diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index 9c4c8b3b72a39..74a2f5c227d9a 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -582,7 +582,7 @@ struct chrono_formatter {
   void write(Rep value, int width) {
     write_sign();
     if (isnan(value)) return write_nan();
-    uint32_or_64_t<int> n = to_unsigned(
+    uint32_or_64_or_128_t<int> n = to_unsigned(
         to_nonnegative_int(value, (std::numeric_limits<int>::max)()));
     int num_digits = internal::count_digits(n);
     if (width > num_digits) out = std::fill_n(out, width - num_digits, '0');
diff --git a/include/fmt/core.h b/include/fmt/core.h
index bda5bfc8c6d7d..63eaa951fe1fc 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -643,6 +643,8 @@ enum type {
   uint_type,
   long_long_type,
   ulong_long_type,
+  int128_type,
+  uint128_type,
   bool_type,
   char_type,
   last_integer_type = char_type,
@@ -669,6 +671,13 @@ FMT_TYPE_CONSTANT(int, int_type);
 FMT_TYPE_CONSTANT(unsigned, uint_type);
 FMT_TYPE_CONSTANT(long long, long_long_type);
 FMT_TYPE_CONSTANT(unsigned long long, ulong_long_type);
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+FMT_TYPE_CONSTANT(__int128, int128_type);
+FMT_TYPE_CONSTANT(unsigned __int128, uint128_type);
+#pragma GCC diagnostic pop
+#endif
 FMT_TYPE_CONSTANT(bool, bool_type);
 FMT_TYPE_CONSTANT(Char, char_type);
 FMT_TYPE_CONSTANT(double, double_type);
@@ -708,6 +717,13 @@ template <typename Context> class value {
     unsigned uint_value;
     long long long_long_value;
     unsigned long long ulong_long_value;
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+    __int128 int128_value;
+    unsigned __int128 uint128_value;
+#pragma GCC diagnostic pop
+#endif
     bool bool_value;
     char_type char_value;
     double double_value;
@@ -722,6 +738,13 @@ template <typename Context> class value {
   FMT_CONSTEXPR value(unsigned val) : uint_value(val) {}
   value(long long val) : long_long_value(val) {}
   value(unsigned long long val) : ulong_long_value(val) {}
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+  value(__int128 val) : int128_value(val) {}
+  value(unsigned __int128 val) : uint128_value(val) {}
+#pragma GCC diagnostic pop
+#endif
   value(double val) : double_value(val) {}
   value(long double val) : long_double_value(val) {}
   value(bool val) : bool_value(val) {}
@@ -781,6 +804,13 @@ template <typename Context> struct arg_mapper {
   FMT_CONSTEXPR ulong_type map(unsigned long val) { return val; }
   FMT_CONSTEXPR long long map(long long val) { return val; }
   FMT_CONSTEXPR unsigned long long map(unsigned long long val) { return val; }
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+  FMT_CONSTEXPR __int128 map(__int128 val) { return val; }
+  FMT_CONSTEXPR unsigned __int128 map(unsigned __int128 val) { return val; }
+#pragma GCC diagnostic pop
+#endif
   FMT_CONSTEXPR bool map(bool val) { return val; }
 
   template <typename T, FMT_ENABLE_IF(is_char<T>::value)>
@@ -941,6 +971,12 @@ FMT_CONSTEXPR auto visit_format_arg(Visitor&& vis,
     return vis(arg.value_.long_long_value);
   case internal::ulong_long_type:
     return vis(arg.value_.ulong_long_value);
+#if FMT_USE_INT128
+  case internal::int128_type:
+    return vis(arg.value_.int128_value);
+  case internal::uint128_type:
+    return vis(arg.value_.uint128_value);
+#endif
   case internal::bool_type:
     return vis(arg.value_.bool_value);
   case internal::char_type:
diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index 3fbb8060010c6..f6e044b674895 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -158,7 +158,7 @@ FMT_FUNC void format_error_code(internal::buffer<char>& out, int error_code,
   static const char ERROR_STR[] = "error ";
   // Subtract 2 to account for terminating null characters in SEP and ERROR_STR.
   std::size_t error_code_size = sizeof(SEP) + sizeof(ERROR_STR) - 2;
-  auto abs_value = static_cast<uint32_or_64_t<int>>(error_code);
+  auto abs_value = static_cast<uint32_or_64_or_128_t<int>>(error_code);
   if (internal::is_negative(error_code)) {
     abs_value = 0 - abs_value;
     ++error_code_size;
diff --git a/include/fmt/format.h b/include/fmt/format.h
index d43435bac09a9..a09799bc8d31e 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -636,11 +636,23 @@ FMT_CONSTEXPR bool is_negative(T) {
   return false;
 }
 
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+// Smallest of uint32_t, uint64_t, unsigned __int128 that is large enough to represent all
+// values of T.
+template <typename T>
+using uint32_or_64_or_128_t =
+    conditional_t<std::numeric_limits<T>::digits <= 32, uint32_t,
+      conditional_t<std::numeric_limits<T>::digits <= 64, uint64_t, unsigned __int128>>;
+#pragma GCC diagnostic pop
+#else
 // Smallest of uint32_t and uint64_t that is large enough to represent all
 // values of T.
 template <typename T>
-using uint32_or_64_t =
+using uint32_or_64_or_128_t =
     conditional_t<std::numeric_limits<T>::digits <= 32, uint32_t, uint64_t>;
+#endif
 
 // Static data is placed in this class template for the header-only config.
 template <typename T = void> struct FMT_EXTERN_TEMPLATE_API basic_data {
@@ -689,6 +701,26 @@ inline int count_digits(uint64_t n) {
 }
 #endif
 
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+inline int count_digits(unsigned __int128 n) {
+  int count = 1;
+  for (;;) {
+    // Integer division is slow so do it for a group of four digits instead
+    // of for every digit. The idea comes from the talk by Alexandrescu
+    // "Three Optimization Tips for C++". See speed-test for a comparison.
+    if (n < 10) return count;
+    if (n < 100) return count + 1;
+    if (n < 1000) return count + 2;
+    if (n < 10000) return count + 3;
+    n /= 10000u;
+    count += 4;
+  }
+}
+#pragma GCC diagnostic pop
+#endif
+
 // Counts the number of digits in n. BITS = log2(radix).
 template <unsigned BITS, typename UInt> inline int count_digits(UInt n) {
   int num_digits = 0;
@@ -818,12 +850,32 @@ inline Char* format_decimal(Char* buffer, UInt value, int num_digits,
   return end;
 }
 
+template <typename Int>
+constexpr int digits10() noexcept {
+  return std::numeric_limits<Int>::digits10;
+}
+
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+template <>
+constexpr int digits10<__int128>() noexcept {
+  return 38;
+}
+
+template <>
+constexpr int digits10<unsigned __int128>() noexcept {
+  return 38;
+}
+#pragma GCC diagnostic pop
+#endif
+
 template <typename Char, typename UInt, typename Iterator, typename F>
 inline Iterator format_decimal(Iterator out, UInt value, int num_digits,
                                F add_thousands_sep) {
   FMT_ASSERT(num_digits >= 0, "invalid digit count");
   // Buffer should be large enough to hold all digits (<= digits10 + 1).
-  enum { max_size = std::numeric_limits<UInt>::digits10 + 1 };
+  enum { max_size = digits10<UInt>() + 1 };
   Char buffer[max_size + max_size / 3];
   auto end = format_decimal(buffer, value, num_digits, add_thousands_sep);
   return internal::copy_str<Char>(buffer, end, out);
@@ -1324,7 +1376,7 @@ template <typename Range> class basic_writer {
 
   // Writes a decimal integer.
   template <typename Int> void write_decimal(Int value) {
-    auto abs_value = static_cast<uint32_or_64_t<Int>>(value);
+    auto abs_value = static_cast<uint32_or_64_or_128_t<Int>>(value);
     bool is_negative = internal::is_negative(value);
     if (is_negative) abs_value = 0 - abs_value;
     int num_digits = internal::count_digits(abs_value);
@@ -1336,7 +1388,7 @@ template <typename Range> class basic_writer {
 
   // The handle_int_type_spec handler that writes an integer.
   template <typename Int, typename Specs> struct int_writer {
-    using unsigned_type = uint32_or_64_t<Int>;
+    using unsigned_type = uint32_or_64_or_128_t<Int>;
 
     basic_writer<Range>& writer;
     const Specs& specs;
@@ -1608,10 +1660,22 @@ template <typename Range> class basic_writer {
   void write(int value) { write_decimal(value); }
   void write(long value) { write_decimal(value); }
   void write(long long value) { write_decimal(value); }
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+  void write(__int128 value) { write_decimal(value); }
+#pragma GCC diagnostic pop
+#endif
 
   void write(unsigned value) { write_decimal(value); }
   void write(unsigned long value) { write_decimal(value); }
   void write(unsigned long long value) { write_decimal(value); }
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+  void write(unsigned __int128 value) { write_decimal(value); }
+#pragma GCC diagnostic pop
+#endif
 
   // Writes a formatted integer.
   template <typename T, typename Spec>
@@ -1694,6 +1758,20 @@ template <typename Range> class basic_writer {
 
 using writer = basic_writer<buffer_range<char>>;
 
+template <typename T>
+struct is_integral_s : std::is_integral<T> {};
+
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+template <>
+struct is_integral_s<__int128> : std::true_type {};
+
+template <>
+struct is_integral_s<unsigned __int128> : std::true_type {};
+#pragma GCC diagnostic pop
+#endif
+
 template <typename Range, typename ErrorHandler = internal::error_handler>
 class arg_formatter_base {
  public:
@@ -1756,7 +1834,7 @@ class arg_formatter_base {
     return out();
   }
 
-  template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+  template <typename T, FMT_ENABLE_IF(is_integral_s<T>::value)>
   iterator operator()(T value) {
     if (specs_)
       writer_.write_int(value, *specs_);
@@ -1888,7 +1966,7 @@ template <typename Context> class custom_formatter {
 
 template <typename T>
 using is_integer =
-    bool_constant<std::is_integral<T>::value && !std::is_same<T, bool>::value &&
+    bool_constant<is_integral_s<T>::value && !std::is_same<T, bool>::value &&
                   !std::is_same<T, char>::value &&
                   !std::is_same<T, wchar_t>::value>;
 
@@ -2947,6 +3025,8 @@ struct formatter<T, Char,
     case internal::uint_type:
     case internal::long_long_type:
     case internal::ulong_long_type:
+    case internal::int128_type:
+    case internal::uint128_type:
     case internal::bool_type:
       handle_int_type_spec(specs_.type,
                            internal::int_type_checker<decltype(eh)>(eh));
diff --git a/include/fmt/printf.h b/include/fmt/printf.h
index a0c2d14a593d4..c67424a46f7cc 100644
--- a/include/fmt/printf.h
+++ b/include/fmt/printf.h
@@ -158,7 +158,7 @@ template <typename Char> class printf_width_handler {
 
   template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
   unsigned operator()(T value) {
-    auto width = static_cast<uint32_or_64_t<T>>(value);
+    auto width = static_cast<uint32_or_64_or_128_t<T>>(value);
     if (internal::is_negative(value)) {
       specs_.align = align::left;
       width = 0 - width;
diff --git a/test/format-impl-test.cc b/test/format-impl-test.cc
index 2017283b40da9..5ee663250774a 100644
--- a/test/format-impl-test.cc
+++ b/test/format-impl-test.cc
@@ -149,6 +149,17 @@ template <typename T> struct value_extractor {
   template <typename U> FMT_NORETURN T operator()(U) {
     throw std::runtime_error(fmt::format("invalid type {}", typeid(U).name()));
   }
+
+#ifdef __apple_build_version__
+  // Apple Clang does not define typeid for __int128.
+  FMT_NORETURN T operator()(__int128) {
+    throw std::runtime_error(fmt::format("invalid type {}", "__int128"));
+  }
+
+  FMT_NORETURN T operator()(unsigned __int128) {
+    throw std::runtime_error(fmt::format("invalid type {}", "unsigned __int128"));
+  }
+#endif
 };
 
 TEST(FormatTest, ArgConverter) {
diff --git a/test/format-test.cc b/test/format-test.cc
index 419ccd2222c4a..83895775bb4ce 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1923,7 +1923,14 @@ using buffer_range = fmt::buffer_range<char>;
 class mock_arg_formatter
     : public fmt::internal::arg_formatter_base<buffer_range> {
  private:
+#if FMT_USE_INT128
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+  MOCK_METHOD1(call, void(__int128 value));
+#pragma GCC diagnostic pop
+#else
   MOCK_METHOD1(call, void(long long value));
+#endif
 
  public:
   typedef fmt::internal::arg_formatter_base<buffer_range> base;
@@ -1936,14 +1943,14 @@ class mock_arg_formatter
   }
 
   template <typename T>
-  typename std::enable_if<std::is_integral<T>::value, iterator>::type
+  typename std::enable_if<fmt::internal::is_integral_s<T>::value, iterator>::type
   operator()(T value) {
     call(value);
     return base::operator()(value);
   }
 
   template <typename T>
-  typename std::enable_if<!std::is_integral<T>::value, iterator>::type
+  typename std::enable_if<!fmt::internal::is_integral_s<T>::value, iterator>::type
   operator()(T value) {
     return base::operator()(value);
   }