diff --git a/tools/test-proxy/Azure.Sdk.Tools.TestProxy.Tests/SanitizerTests.cs b/tools/test-proxy/Azure.Sdk.Tools.TestProxy.Tests/SanitizerTests.cs
index ca9d1688f42..2ab312b1ed1 100644
--- a/tools/test-proxy/Azure.Sdk.Tools.TestProxy.Tests/SanitizerTests.cs
+++ b/tools/test-proxy/Azure.Sdk.Tools.TestProxy.Tests/SanitizerTests.cs
@@ -1,4 +1,4 @@
-using Azure.Sdk.Tools.TestProxy.Common;
+using Azure.Sdk.Tools.TestProxy.Common;
 using Azure.Sdk.Tools.TestProxy.Common.Exceptions;
 using Azure.Sdk.Tools.TestProxy.Sanitizers;
 using Microsoft.AspNetCore.Http;
@@ -277,6 +277,35 @@ public void HeaderRegexSanitizerSimpleReplace()
             Assert.StartsWith("https://fakeaccount.table.core.windows.net", testValue);
         }
 
+
+        [Fact]
+        public void HeaderRegexSanitizerMultipartReplace()
+        {
+            var session = TestHelpers.LoadRecordSession("Test.RecordEntries/multipart_header.json");
+            var targetEntry = session.Session.Entries[0];
+            var targetKey = "Cookie";
+
+            var headerRegexSanitizer = new HeaderRegexSanitizer(targetKey, value: "REDACTED", regex: "SuperDifferent");
+            session.Session.Sanitize(headerRegexSanitizer);
+
+            Assert.Equal("REDACTEDCookie", targetEntry.Request.Headers[targetKey][0]);
+            Assert.Equal("KindaDifferentCookie", targetEntry.Request.Headers[targetKey][1]);
+        }
+
+        [Fact]
+        public void HeaderRegexSanitizerMultipartReplaceLatterOnly()
+        {
+            var session = TestHelpers.LoadRecordSession("Test.RecordEntries/multipart_header.json");
+            var targetEntry = session.Session.Entries[0];
+            var targetKey = "Cookie";
+
+            var headerRegexSanitizer = new HeaderRegexSanitizer(targetKey, value: "REDACTED", regex: "KindaDifferent");
+            session.Session.Sanitize(headerRegexSanitizer);
+
+            Assert.Equal("SuperDifferentCookie", targetEntry.Request.Headers[targetKey][0]);
+            Assert.Equal("REDACTEDCookie", targetEntry.Request.Headers[targetKey][1]);
+        }
+
         [Fact]
         public void HeaderRegexSanitizerGroupedRegexReplace()
         {
diff --git a/tools/test-proxy/Azure.Sdk.Tools.TestProxy.Tests/Test.RecordEntries/multipart_header.json b/tools/test-proxy/Azure.Sdk.Tools.TestProxy.Tests/Test.RecordEntries/multipart_header.json
new file mode 100644
index 00000000000..f5618b28040
--- /dev/null
+++ b/tools/test-proxy/Azure.Sdk.Tools.TestProxy.Tests/Test.RecordEntries/multipart_header.json
@@ -0,0 +1,39 @@
+{
+  "Entries": [
+    {
+      "RequestUri": "http://host.docker.internal:8080/sample_response",
+      "RequestMethod": "GET",
+      "RequestHeaders": {
+        "Accept": "*/*",
+        "Accept-Encoding": "gzip, deflate, br",
+        "Accept-Language": "en-US",
+        "Connection": "keep-alive",
+        "Content-Length":  "11",
+        "If-None-Match": "W/\u0022d-ua9UN3rfp1JzmLYtPUOjtDq\u002B3co\u0022",
+        "Referer": "http://localhost:9328/",
+        "sec-ch-ua": "",
+        "sec-ch-ua-mobile": "?0",
+        "sec-ch-ua-platform": "",
+        "Sec-Fetch-Dest": "empty",
+        "Sec-Fetch-Mode": "cors",
+        "Sec-Fetch-Site": "same-site",
+        "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/93.0.4577.0 Safari/537.36",
+        "x-ms-client-request-id": "7c65aa72-6b12-4b60-bf3b-7a85c174e2ba",
+        "x-ms-useragent": "core-rest-pipeline/1.3.3 OS/Win32",
+        "Cookie": [
+          "SuperDifferentCookie",
+          "KindaDifferentCookie"
+        ]
+      },
+      "RequestBody": "RequestBody=",
+      "StatusCode": 304,
+      "ResponseHeaders": {
+        "Connection": "keep-alive",
+        "Keep-Alive": "timeout=5",
+        "X-Powered-By": "Express"
+      },
+      "ResponseBody": null
+    }
+  ],
+  "Variables": {}
+}
diff --git a/tools/test-proxy/Azure.Sdk.Tools.TestProxy/Sanitizers/HeaderRegexSanitizer.cs b/tools/test-proxy/Azure.Sdk.Tools.TestProxy/Sanitizers/HeaderRegexSanitizer.cs
index ae6880de930..fceae19b33b 100644
--- a/tools/test-proxy/Azure.Sdk.Tools.TestProxy/Sanitizers/HeaderRegexSanitizer.cs
+++ b/tools/test-proxy/Azure.Sdk.Tools.TestProxy/Sanitizers/HeaderRegexSanitizer.cs
@@ -1,5 +1,6 @@
-using Azure.Sdk.Tools.TestProxy.Common;
+using Azure.Sdk.Tools.TestProxy.Common;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace Azure.Sdk.Tools.TestProxy.Sanitizers
 {
@@ -47,11 +48,7 @@ public override void SanitizeHeaders(IDictionary<string, string[]> headers)
                 // We do this because letting .NET split and then reassemble header values introduces a space into the header itself
                 // Ex: "application/json;odata=minimalmetadata" with .NET default header parsing becomes "application/json; odata=minimalmetadata"
                 // Given this breaks signature verification, we have to avoid it.
-                var originalValue = headers[_targetKey][0];
-
-                var replacement = StringSanitizer.SanitizeValue(originalValue, _newValue, _regexValue, _groupForReplace);
-
-                headers[_targetKey] = new string[] { replacement };
+                headers[_targetKey] = headers[_targetKey].Select(x => StringSanitizer.SanitizeValue(x, _newValue, _regexValue, _groupForReplace)).ToArray();
             }
         }
     }
diff --git a/tools/test-proxy/Azure.Sdk.Tools.TestProxy/Sanitizers/HeaderStringSanitizer.cs b/tools/test-proxy/Azure.Sdk.Tools.TestProxy/Sanitizers/HeaderStringSanitizer.cs
index 9b8472aba76..8655f249b7e 100644
--- a/tools/test-proxy/Azure.Sdk.Tools.TestProxy/Sanitizers/HeaderStringSanitizer.cs
+++ b/tools/test-proxy/Azure.Sdk.Tools.TestProxy/Sanitizers/HeaderStringSanitizer.cs
@@ -1,5 +1,6 @@
-using Azure.Sdk.Tools.TestProxy.Common;
+using Azure.Sdk.Tools.TestProxy.Common;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace Azure.Sdk.Tools.TestProxy.Sanitizers
 {
@@ -39,11 +40,7 @@ public override void SanitizeHeaders(IDictionary<string, string[]> headers)
                 // We do this because letting .NET split and then reassemble header values introduces a space into the header itself
                 // Ex: "application/json;odata=minimalmetadata" with .NET default header parsing becomes "application/json; odata=minimalmetadata"
                 // Given this breaks signature verification, we have to avoid it.
-                var originalValue = headers[_targetKey][0];
-
-                var replacement = StringSanitizer.ReplaceValue(inputValue: originalValue, targetValue: _targetValue, replacementValue: _newValue);
-
-                headers[_targetKey] = new string[] { replacement };
+                headers[_targetKey] = headers[_targetKey].Select(x => StringSanitizer.ReplaceValue(inputValue: x, targetValue: _targetValue, replacementValue: _newValue)).ToArray();
             }
         }
     }