diff --git a/src/libraries/System.Memory/ref/System.Memory.Forwards.cs b/src/libraries/System.Memory/ref/System.Memory.Forwards.cs
index 07916d433d445..e79102cfcd350 100644
--- a/src/libraries/System.Memory/ref/System.Memory.Forwards.cs
+++ b/src/libraries/System.Memory/ref/System.Memory.Forwards.cs
@@ -10,3 +10,4 @@
 [assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Buffers.MemoryHandle))]
 [assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Buffers.MemoryManager<>))]
 [assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Buffers.OperationStatus))]
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Buffers.Text.Base64))]
diff --git a/src/libraries/System.Memory/ref/System.Memory.cs b/src/libraries/System.Memory/ref/System.Memory.cs
index 648959e958d70..82076983e6062 100644
--- a/src/libraries/System.Memory/ref/System.Memory.cs
+++ b/src/libraries/System.Memory/ref/System.Memory.cs
@@ -474,15 +474,6 @@ public static void WriteUInt64LittleEndian(System.Span<byte> destination, ulong
 }
 namespace System.Buffers.Text
 {
-    public static partial class Base64
-    {
-        public static System.Buffers.OperationStatus DecodeFromUtf8(System.ReadOnlySpan<byte> utf8, System.Span<byte> bytes, out int bytesConsumed, out int bytesWritten, bool isFinalBlock = true) { throw null; }
-        public static System.Buffers.OperationStatus DecodeFromUtf8InPlace(System.Span<byte> buffer, out int bytesWritten) { throw null; }
-        public static System.Buffers.OperationStatus EncodeToUtf8(System.ReadOnlySpan<byte> bytes, System.Span<byte> utf8, out int bytesConsumed, out int bytesWritten, bool isFinalBlock = true) { throw null; }
-        public static System.Buffers.OperationStatus EncodeToUtf8InPlace(System.Span<byte> buffer, int dataLength, out int bytesWritten) { throw null; }
-        public static int GetMaxDecodedFromUtf8Length(int length) { throw null; }
-        public static int GetMaxEncodedToUtf8Length(int length) { throw null; }
-    }
     public static partial class Utf8Formatter
     {
         public static bool TryFormat(bool value, System.Span<byte> destination, out int bytesWritten, System.Buffers.StandardFormat format = default(System.Buffers.StandardFormat)) { throw null; }
diff --git a/src/libraries/System.Memory/src/System.Memory.csproj b/src/libraries/System.Memory/src/System.Memory.csproj
index 7294197f7a962..a250bb20793b8 100644
--- a/src/libraries/System.Memory/src/System.Memory.csproj
+++ b/src/libraries/System.Memory/src/System.Memory.csproj
@@ -25,9 +25,6 @@
     <Compile Include="System\Buffers\SequenceReader.cs" />
     <Compile Include="System\Buffers\SequenceReader.Search.cs" />
     <Compile Include="System\Buffers\SequenceReaderExtensions.Binary.cs" />
-    <Compile Include="System\Buffers\Text\Base64.cs" />
-    <Compile Include="System\Buffers\Text\Base64Decoder.cs" />
-    <Compile Include="System\Buffers\Text\Base64Encoder.cs" />
     <Compile Include="System\Runtime\InteropServices\SequenceMarshal.cs" />
     <Compile Include="System\Text\EncodingExtensions.cs" />
   </ItemGroup>
diff --git a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
index d978ef0b8c488..e6842b9549125 100644
--- a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
+++ b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
@@ -114,6 +114,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\MemoryManager.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\OperationStatus.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\StandardFormat.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Base64.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Base64Encoder.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Base64Decoder.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\FormattingHelpers.CountDigits.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Constants.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\FormattingHelpers.cs" />
diff --git a/src/libraries/System.Memory/src/System/Buffers/Text/Base64.cs b/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64.cs
similarity index 100%
rename from src/libraries/System.Memory/src/System/Buffers/Text/Base64.cs
rename to src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64.cs
diff --git a/src/libraries/System.Memory/src/System/Buffers/Text/Base64Decoder.cs b/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs
similarity index 100%
rename from src/libraries/System.Memory/src/System/Buffers/Text/Base64Decoder.cs
rename to src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs
diff --git a/src/libraries/System.Memory/src/System/Buffers/Text/Base64Encoder.cs b/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Encoder.cs
similarity index 100%
rename from src/libraries/System.Memory/src/System/Buffers/Text/Base64Encoder.cs
rename to src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Encoder.cs
diff --git a/src/libraries/System.Private.CoreLib/src/System/Convert.cs b/src/libraries/System.Private.CoreLib/src/System/Convert.cs
index 5b212556c684e..34a4dd112bc72 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Convert.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Convert.cs
@@ -6,6 +6,9 @@
 using System.Globalization;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Buffers;
+using System.Buffers.Text;
+using System.Text;
 
 namespace System
 {
@@ -2336,7 +2339,30 @@ public static string ToBase64String(ReadOnlySpan<byte> bytes, Base64FormattingOp
             }
 
             bool insertLineBreaks = (options == Base64FormattingOptions.InsertLineBreaks);
-            string result = string.FastAllocateString(ToBase64_CalculateAndValidateOutputLength(bytes.Length, insertLineBreaks));
+            int outputLength = ToBase64_CalculateAndValidateOutputLength(bytes.Length, insertLineBreaks);
+
+            if (!insertLineBreaks && bytes.Length >= 64)
+            {
+                // For large inputs it's faster to allocate a temp buffer and call UTF8 version
+                // which is then extended to UTF8 via Latin1.GetString (base64 is always ASCI)
+                [MethodImpl(MethodImplOptions.NoInlining)]
+                static string ToBase64StringLargeInputs(ReadOnlySpan<byte> data, int outputLen)
+                {
+                    byte[]? rentedBytes = null;
+                    Span<byte> utf8buffer = outputLen <= 256 ? stackalloc byte[256] : (rentedBytes = ArrayPool<byte>.Shared.Rent(outputLen));
+                    OperationStatus status = Base64.EncodeToUtf8(data, utf8buffer, out int _, out int bytesWritten);
+                    Debug.Assert(status == OperationStatus.Done && bytesWritten == outputLen);
+                    string result = Encoding.Latin1.GetString(utf8buffer.Slice(0, outputLen));
+                    if (rentedBytes != null)
+                    {
+                        ArrayPool<byte>.Shared.Return(rentedBytes);
+                    }
+                    return result;
+                }
+                return ToBase64StringLargeInputs(bytes, outputLength);
+            }
+
+            string result = string.FastAllocateString(outputLength);
 
             unsafe
             {
diff --git a/src/libraries/System.Runtime.Extensions/tests/System/Convert.ToBase64String.cs b/src/libraries/System.Runtime.Extensions/tests/System/Convert.ToBase64String.cs
index c3c5b55693562..d8a34d7816db6 100644
--- a/src/libraries/System.Runtime.Extensions/tests/System/Convert.ToBase64String.cs
+++ b/src/libraries/System.Runtime.Extensions/tests/System/Convert.ToBase64String.cs
@@ -2,6 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using Xunit;
+using System.Collections.Generic;
+using System.Text;
+using System.Linq;
 
 namespace System.Tests
 {
@@ -56,5 +59,91 @@ public static void InvalidLength()
             Assert.Throws<ArgumentOutOfRangeException>(() => Convert.ToBase64String(inputBytes, 0, inputBytes.Length + 1));
             Assert.Throws<ArgumentOutOfRangeException>(() => Convert.ToBase64String(inputBytes, 1, inputBytes.Length));
         }
+
+        public static IEnumerable<object[]> ConvertToBase64StringTests_TestData()
+        {
+            yield return new object[] { Enumerable.Range(0, 0).Select(i => (byte)i).ToArray(), "" };
+            yield return new object[] { Enumerable.Range(0, 1).Select(i => (byte)i).ToArray(), "AA==" };
+            yield return new object[] { Enumerable.Range(0, 2).Select(i => (byte)i).ToArray(), "AAE=" };
+            yield return new object[] { Enumerable.Range(0, 3).Select(i => (byte)i).ToArray(), "AAEC" };
+            yield return new object[] { Enumerable.Range(0, 4).Select(i => (byte)i).ToArray(), "AAECAw==" };
+            yield return new object[] { Enumerable.Range(0, 5).Select(i => (byte)i).ToArray(), "AAECAwQ=" };
+            yield return new object[] { Enumerable.Range(0, 6).Select(i => (byte)i).ToArray(), "AAECAwQF" };
+            yield return new object[] { Enumerable.Range(0, 7).Select(i => (byte)i).ToArray(), "AAECAwQFBg==" };
+            yield return new object[] { Enumerable.Range(0, 8).Select(i => (byte)i).ToArray(), "AAECAwQFBgc=" };
+            yield return new object[] { Enumerable.Range(0, 9).Select(i => (byte)i).ToArray(), "AAECAwQFBgcI" };
+            yield return new object[] { Enumerable.Range(0, 10).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQ==" };
+            yield return new object[] { Enumerable.Range(0, 11).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQo=" };
+            yield return new object[] { Enumerable.Range(0, 12).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoL" };
+            yield return new object[] { Enumerable.Range(0, 13).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA==" };
+            yield return new object[] { Enumerable.Range(0, 14).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0=" };
+            yield return new object[] { Enumerable.Range(0, 15).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0O" };
+            yield return new object[] { Enumerable.Range(0, 16).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODw==" };
+            yield return new object[] { Enumerable.Range(0, 17).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxA=" };
+            yield return new object[] { Enumerable.Range(0, 18).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAR" };
+            yield return new object[] { Enumerable.Range(0, 19).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREg==" };
+            yield return new object[] { Enumerable.Range(0, 20).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhM=" };
+            yield return new object[] { Enumerable.Range(0, 21).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMU" };
+            yield return new object[] { Enumerable.Range(0, 22).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFQ==" };
+            yield return new object[] { Enumerable.Range(0, 23).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRY=" };
+            yield return new object[] { Enumerable.Range(0, 24).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYX" };
+            yield return new object[] { Enumerable.Range(0, 25).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGA==" };
+            yield return new object[] { Enumerable.Range(0, 26).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBk=" };
+            yield return new object[] { Enumerable.Range(0, 27).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBka" };
+            yield return new object[] { Enumerable.Range(0, 28).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGw==" };
+            yield return new object[] { Enumerable.Range(0, 29).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxw=" };
+            yield return new object[] { Enumerable.Range(0, 30).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwd" };
+            yield return new object[] { Enumerable.Range(0, 31).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHg==" };
+            yield return new object[] { Enumerable.Range(0, 32).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8=" };
+            yield return new object[] { Enumerable.Range(0, 33).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8g" };
+            yield return new object[] { Enumerable.Range(0, 34).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gIQ==" };
+            yield return new object[] { Enumerable.Range(0, 35).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISI=" };
+            yield return new object[] { Enumerable.Range(0, 36).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIj" };
+            yield return new object[] { Enumerable.Range(0, 37).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJA==" };
+            yield return new object[] { Enumerable.Range(0, 38).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCU=" };
+            yield return new object[] { Enumerable.Range(0, 39).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUm" };
+            yield return new object[] { Enumerable.Range(0, 40).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJw==" };
+            yield return new object[] { Enumerable.Range(0, 41).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJyg=" };
+            yield return new object[] { Enumerable.Range(0, 42).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygp" };
+            yield return new object[] { Enumerable.Range(0, 43).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKg==" };
+            yield return new object[] { Enumerable.Range(0, 44).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKis=" };
+            yield return new object[] { Enumerable.Range(0, 45).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKiss" };
+            yield return new object[] { Enumerable.Range(0, 46).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLQ==" };
+            yield return new object[] { Enumerable.Range(0, 47).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4=" };
+            yield return new object[] { Enumerable.Range(0, 48).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4v" };
+            yield return new object[] { Enumerable.Range(0, 49).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMA==" };
+            yield return new object[] { Enumerable.Range(0, 50).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDE=" };
+            yield return new object[] { Enumerable.Range(0, 51).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEy" };
+            yield return new object[] { Enumerable.Range(0, 52).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMw==" };
+            yield return new object[] { Enumerable.Range(0, 53).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ=" };
+            yield return new object[] { Enumerable.Range(0, 54).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1" };
+            yield return new object[] { Enumerable.Range(0, 55).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Ng==" };
+            yield return new object[] { Enumerable.Range(0, 56).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc=" };
+            yield return new object[] { Enumerable.Range(0, 57).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4" };
+            yield return new object[] { Enumerable.Range(0, 58).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OQ==" };
+            yield return new object[] { Enumerable.Range(0, 59).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo=" };
+            yield return new object[] { Enumerable.Range(0, 60).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7" };
+            yield return new object[] { Enumerable.Range(0, 61).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7PA==" };
+            yield return new object[] { Enumerable.Range(0, 62).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7PD0=" };
+            yield return new object[] { Enumerable.Range(0, 63).Select(i => (byte)i).ToArray(), "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7PD0+" };
+            yield return new object[] { Encoding.Unicode.GetBytes("aaaabbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccdd"), "YQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAA==" };
+            yield return new object[] { Encoding.Unicode.GetBytes("vbnmbbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccddx"), "dgBiAG4AbQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAHgA" };
+            yield return new object[] { Encoding.Unicode.GetBytes("rrrrbbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccdd\0"), "cgByAHIAcgBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAAAA" };
+            yield return new object[] { Encoding.Unicode.GetBytes("uuuubbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccdd\0feffe"), "dQB1AHUAdQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAAAAZgBlAGYAZgBlAA==" };
+            yield return new object[] { Encoding.Unicode.GetBytes("kkkkkbbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccddxприветмир你好世界"), "awBrAGsAawBrAGIAYgBiAGIAYwBjAGMAYwBkAGQAZABkAGQAZABkAGUAZQBlAGUAZQBhAGEAYQBhAGIAYgBiAGIAYwBjAGMAYwBkAGQAZABkAGQAZABkAGUAZQBlAGUAZQBhAGEAYQBhAGIAYgBiAGIAYwBjAGMAYwBkAGQAeAA/BEAEOAQyBDUEQgQ8BDgEQARgT31ZFk5MdQ==" };
+            yield return new object[] { Encoding.Unicode.GetBytes(",,,,bbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccddxприветмир你好世界ddddeeeeea"), "LAAsACwALABiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAHgAPwRABDgEMgQ1BEIEPAQ4BEAEYE99WRZOTHVkAGQAZABkAGUAZQBlAGUAZQBhAA==" };
+            yield return new object[] { Encoding.Unicode.GetBytes("____bbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccddaaaabbbbccccdddddddeeeeeaaaabbbbccccdcccd"), "XwBfAF8AXwBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGEAYQBhAGEAYgBiAGIAYgBjAGMAYwBjAGQAZABkAGQAZABkAGQAZQBlAGUAZQBlAGEAYQBhAGEAYgBiAGIAYgBjAGMAYwBjAGQAYwBjAGMAZAA=" };
+            yield return new object[] { Encoding.Unicode.GetBytes("    bbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccddaaaabbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccd"), "IAAgACAAIABiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGEAYQBhAGEAYgBiAGIAYgBjAGMAYwBjAGQAZABkAGQAZABkAGQAZQBlAGUAZQBlAGEAYQBhAGEAYgBiAGIAYgBjAGMAYwBjAGQAZABkAGQAZABkAGQAZQBlAGUAZQBlAGEAYQBhAGEAYgBiAGIAYgBjAGMAYwBjAGQA" };
+            yield return new object[] { Encoding.Unicode.GetBytes("\0\0bbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccddaaaabbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccddx"), "AAAAAGIAYgBiAGIAYwBjAGMAYwBkAGQAZABkAGQAZABkAGUAZQBlAGUAZQBhAGEAYQBhAGIAYgBiAGIAYwBjAGMAYwBkAGQAZABkAGQAZABkAGUAZQBlAGUAZQBhAGEAYQBhAGIAYgBiAGIAYwBjAGMAYwBkAGQAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAHgA" };
+            yield return new object[] { Encoding.Unicode.GetBytes("eeeebbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccdgggdaaaabbbbccccdddddddeeeeeaaaabbbbccccdddddddeeeeeaaaabbbbccccddx"), "ZQBlAGUAZQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABkAGQAZABkAGQAZABlAGUAZQBlAGUAYQBhAGEAYQBiAGIAYgBiAGMAYwBjAGMAZABnAGcAZwBkAGEAYQBhAGEAYgBiAGIAYgBjAGMAYwBjAGQAZABkAGQAZABkAGQAZQBlAGUAZQBlAGEAYQBhAGEAYgBiAGIAYgBjAGMAYwBjAGQAZABkAGQAZABkAGQAZQBlAGUAZQBlAGEAYQBhAGEAYgBiAGIAYgBjAGMAYwBjAGQAZAB4AA==" };
+        }
+
+        
+        [Theory]
+        [MemberData(nameof(ConvertToBase64StringTests_TestData))]
+        public static void ConvertToBase64String(byte[] inputBytes, string expectedBase64)
+        {
+            Assert.Equal(expectedBase64, Convert.ToBase64String(inputBytes));
+        }
     }
 }
diff --git a/src/libraries/System.Runtime/ref/System.Runtime.cs b/src/libraries/System.Runtime/ref/System.Runtime.cs
index 07abb8a23199f..9c0b990aa5f00 100644
--- a/src/libraries/System.Runtime/ref/System.Runtime.cs
+++ b/src/libraries/System.Runtime/ref/System.Runtime.cs
@@ -7315,6 +7315,18 @@ public enum OperationStatus
     public delegate void ReadOnlySpanAction<T, in TArg>(System.ReadOnlySpan<T> span, TArg arg);
     public delegate void SpanAction<T, in TArg>(System.Span<T> span, TArg arg);
 }
+namespace System.Buffers.Text
+{
+    public static partial class Base64
+    {
+        public static System.Buffers.OperationStatus DecodeFromUtf8(System.ReadOnlySpan<byte> utf8, System.Span<byte> bytes, out int bytesConsumed, out int bytesWritten, bool isFinalBlock = true) { throw null; }
+        public static System.Buffers.OperationStatus DecodeFromUtf8InPlace(System.Span<byte> buffer, out int bytesWritten) { throw null; }
+        public static System.Buffers.OperationStatus EncodeToUtf8(System.ReadOnlySpan<byte> bytes, System.Span<byte> utf8, out int bytesConsumed, out int bytesWritten, bool isFinalBlock = true) { throw null; }
+        public static System.Buffers.OperationStatus EncodeToUtf8InPlace(System.Span<byte> buffer, int dataLength, out int bytesWritten) { throw null; }
+        public static int GetMaxDecodedFromUtf8Length(int length) { throw null; }
+        public static int GetMaxEncodedToUtf8Length(int length) { throw null; }
+    }
+}
 namespace System.CodeDom.Compiler
 {
     [System.AttributeUsageAttribute(System.AttributeTargets.All, Inherited=false, AllowMultiple=false)]