diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/libc/config/linux/aarch64/entrypoints.txt
@@ -21,6 +21,7 @@
     libc.src.string.strlen
     libc.src.string.strnlen
     libc.src.string.strrchr
+    libc.src.string.strspn
     libc.src.string.strstr
 )
 
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -39,6 +39,7 @@
     libc.src.string.strlen
     libc.src.string.strnlen
     libc.src.string.strrchr
+    libc.src.string.strspn
     libc.src.string.strstr
 
     # sys/mman.h entrypoints
diff --git a/libc/src/string/CMakeLists.txt b/libc/src/string/CMakeLists.txt
--- a/libc/src/string/CMakeLists.txt
+++ b/libc/src/string/CMakeLists.txt
@@ -94,6 +94,14 @@
     strrchr.h
 )
 
+add_entrypoint_object(
+  strspn
+  SRCS
+    strspn.cpp
+  HDRS
+    strspn.h
+)
+
 # Helper to define a function with multiple implementations
 # - Computes flags to satisfy required/rejected features and arch,
 # - Declares an entry point,
diff --git a/libc/src/string/strspn.h b/libc/src/string/strspn.h
new file mode 100644
--- /dev/null
+++ b/libc/src/string/strspn.h
@@ -0,0 +1,20 @@
+//===-- Implementation header for strspn ------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STRING_STRSPN_H
+#define LLVM_LIBC_SRC_STRING_STRSPN_H
+
+#include <stddef.h>
+
+namespace __llvm_libc {
+
+size_t strspn(const char *src, const char *segment);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_STRSPN_H
diff --git a/libc/src/string/strspn.cpp b/libc/src/string/strspn.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/string/strspn.cpp
@@ -0,0 +1,50 @@
+//===-- Implementation of strspn ------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/string/strspn.h"
+
+#include "src/__support/common.h"
+#include <stddef.h>
+
+namespace __llvm_libc {
+
+constexpr size_t exploded_unsigned_char_bit = 8 * sizeof(size_t);
+
+// (256 / 8) / sizeof(size_t).
+constexpr size_t bitset_size = 32 / sizeof(size_t);
+
+static void toggle_bit(size_t *bitset, const size_t ch) {
+  const size_t mask = size_t{1} << (ch % exploded_unsigned_char_bit);
+  bitset[ch / exploded_unsigned_char_bit] |= mask;
+}
+
+static bool contains_bit(const size_t *bitset, const size_t ch) {
+  const size_t mask = size_t{1} << (ch % exploded_unsigned_char_bit);
+  return bitset[ch / exploded_unsigned_char_bit] & mask;
+}
+
+size_t LLVM_LIBC_ENTRYPOINT(strspn)(const char *src, const char *segment) {
+  if (!segment[0])
+    return 0;
+
+  const char *initial = src;
+  if (!segment[1]) { // The segment is a single character.
+    for (; *src == *segment; ++src)
+      ;
+    return src - initial;
+  }
+
+  size_t bitset[bitset_size] = {0};
+  for (; *segment; ++segment)
+    toggle_bit(bitset, *segment);
+  for (; *src && contains_bit(bitset, *src); ++src)
+    ;
+  return src - initial;
+}
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/string/CMakeLists.txt b/libc/test/src/string/CMakeLists.txt
--- a/libc/test/src/string/CMakeLists.txt
+++ b/libc/test/src/string/CMakeLists.txt
@@ -102,6 +102,17 @@
     libc.src.string.strrchr
 )
 
+add_libc_unittest(
+  strspn_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strspn_test.cpp
+  DEPENDS
+    libc.src.string.strspn
+)
+
+
 # Tests all implementations that can run on the host.
 function(add_libc_multi_impl_test name)
   get_property(fq_implementations GLOBAL PROPERTY ${name}_implementations)
diff --git a/libc/test/src/string/strspn_test.cpp b/libc/test/src/string/strspn_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/string/strspn_test.cpp
@@ -0,0 +1,83 @@
+//===-- Unittests for strspn ----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/string/strspn.h"
+
+#include "utils/UnitTest/Test.h"
+
+TEST(StrSpnTest, EmptyStringShouldReturnZero) {
+  // The search should not include the null terminator.
+  EXPECT_EQ(__llvm_libc::strspn("", ""), size_t{0});
+  EXPECT_EQ(__llvm_libc::strspn("_", ""), size_t{0});
+  EXPECT_EQ(__llvm_libc::strspn("", "_"), size_t{0});
+}
+
+TEST(StrSpnTest, ShouldNotMatchAnythingAfterNullTerminator) {
+  const char src[4] = {'a', 'b', '\0', 'c'};
+  EXPECT_EQ(__llvm_libc::strspn(src, "ab"), size_t{2});
+  EXPECT_EQ(__llvm_libc::strspn(src, "c"), size_t{0});
+
+  // Same goes for the segment to be searched for.
+  const char segment[4] = {'1', '2', '\0', '3'};
+  EXPECT_EQ(__llvm_libc::strspn("123", segment), size_t{2});
+}
+
+TEST(StrSpnTest, OnlyFirstCharacterSegmentShouldMatch) {
+  const char *src = "12345";
+  EXPECT_EQ(__llvm_libc::strspn(src, "1"), size_t{1});
+  // Since '1' is not within the segment, the span
+  // size should remain zero.
+  EXPECT_EQ(__llvm_libc::strspn(src, "2"), size_t{0});
+  EXPECT_EQ(__llvm_libc::strspn(src, "3"), size_t{0});
+  EXPECT_EQ(__llvm_libc::strspn(src, "4"), size_t{0});
+  EXPECT_EQ(__llvm_libc::strspn(src, "5"), size_t{0});
+}
+
+TEST(StrSpnTest, UnmatchedCharacterShouldNotCount) {
+  EXPECT_EQ(__llvm_libc::strspn("a", "b"), size_t{0});
+  EXPECT_EQ(__llvm_libc::strspn("abcdef", "1"), size_t{0});
+  EXPECT_EQ(__llvm_libc::strspn("123", "4"), size_t{0});
+}
+
+TEST(StrSpnTest, SequentialCharactersShouldMatch) {
+  const char *src = "abcde";
+  EXPECT_EQ(__llvm_libc::strspn(src, "a"), size_t{1});
+  EXPECT_EQ(__llvm_libc::strspn(src, "ab"), size_t{2});
+  EXPECT_EQ(__llvm_libc::strspn(src, "abc"), size_t{3});
+  EXPECT_EQ(__llvm_libc::strspn(src, "abcd"), size_t{4});
+  EXPECT_EQ(__llvm_libc::strspn(src, "abcde"), size_t{5});
+  // Same thing for when the roles are reversed.
+  EXPECT_EQ(__llvm_libc::strspn("abcde", src), size_t{5});
+  EXPECT_EQ(__llvm_libc::strspn("abcd", src), size_t{4});
+  EXPECT_EQ(__llvm_libc::strspn("abc", src), size_t{3});
+  EXPECT_EQ(__llvm_libc::strspn("ab", src), size_t{2});
+  EXPECT_EQ(__llvm_libc::strspn("a", src), size_t{1});
+}
+
+TEST(StrSpnTest, NonSequentialCharactersShouldNotMatch) {
+  const char *src = "123456789";
+  EXPECT_EQ(__llvm_libc::strspn(src, "_1_abc_2_def_3_"), size_t{3});
+  // Only matches 4 since '5' is not within the span.
+  EXPECT_EQ(__llvm_libc::strspn(src, "67__34abc12"), size_t{4});
+}
+
+TEST(StrSpnTest, ReverseCharactersShouldNotMatch) {
+  EXPECT_EQ(__llvm_libc::strspn("12345", "54321"), size_t{5});
+  EXPECT_EQ(__llvm_libc::strspn("12345", "432"), size_t{0});
+  // Only matches 1 since '2' is not within the span.
+  EXPECT_EQ(__llvm_libc::strspn("12345", "51"), size_t{1});
+}
+
+TEST(StrSpnTest, DuplicatedCharactersToBeSearchedForShouldStillMatch) {
+  // Only a single character, so only 1 match.
+  EXPECT_EQ(__llvm_libc::strspn("a", "aa"), size_t{1});
+  // This should count once for each 'a' in the source string.
+  EXPECT_EQ(__llvm_libc::strspn("aa", "aa"), size_t{2});
+  EXPECT_EQ(__llvm_libc::strspn("aaa", "aa"), size_t{3});
+  EXPECT_EQ(__llvm_libc::strspn("aaaa", "aa"), size_t{4});
+}