diff --git a/crypto/chacha/chacha.c b/crypto/chacha/chacha.c
index 1092b7aa284..a4d88c0a409 100644
--- a/crypto/chacha/chacha.c
+++ b/crypto/chacha/chacha.c
@@ -91,7 +91,25 @@ void CRYPTO_chacha_20(uint8_t *out, const uint8_t *in, size_t in_len,
   }
 #endif
 
-  ChaCha20_ctr32(out, in, in_len, key_ptr, counter_nonce);
+  while (in_len > 0) {
+    // The assembly functions do not have defined overflow behavior. While
+    // overflow is almost always a bug in the caller, we prefer our functions to
+    // behave the same across platforms, so divide into multiple calls to avoid
+    // this case.
+    uint64_t todo = 64 * ((UINT64_C(1) << 32) - counter_nonce[0]);
+    if (todo > in_len) {
+      todo = in_len;
+    }
+
+    ChaCha20_ctr32(out, in, (size_t)todo, key_ptr, counter_nonce);
+    in += todo;
+    out += todo;
+    in_len -= todo;
+
+    // We're either done and will next break out of the loop, or we stopped at
+    // the wraparound point and the counter should continue at zero.
+    counter_nonce[0] = 0;
+  }
 }
 
 #else
diff --git a/crypto/chacha/chacha_test.cc b/crypto/chacha/chacha_test.cc
index 25313cad6d4..00683ce51e1 100644
--- a/crypto/chacha/chacha_test.cc
+++ b/crypto/chacha/chacha_test.cc
@@ -218,8 +218,102 @@ static const uint8_t kOutput[] = {
     0x8f, 0x40, 0xcf, 0x4a,
 };
 
+static uint32_t kOverflowCounter = 0xffffffff;
+
+static const uint8_t kOverflowOutput[] = {
+    0x37, 0x64, 0x38, 0xcb, 0x25, 0x69, 0x2c, 0xf5, 0x88, 0x8a, 0xfe, 0x6d,
+    0x3b, 0x10, 0x07, 0x3c, 0x77, 0xac, 0xcd, 0x1c, 0x0c, 0xa7, 0x17, 0x31,
+    0x1d, 0xc3, 0x81, 0xd1, 0xa5, 0x20, 0x55, 0xea, 0xd3, 0x00, 0xc9, 0x84,
+    0xde, 0xe2, 0xe5, 0x5e, 0x7b, 0x28, 0x28, 0x59, 0x73, 0x3a, 0x8e, 0x57,
+    0x62, 0x18, 0x50, 0x55, 0x97, 0xca, 0x50, 0x3e, 0x8a, 0x84, 0x61, 0x28,
+    0x4c, 0x22, 0x93, 0x50, 0x48, 0x7e, 0x65, 0x78, 0x06, 0x5a, 0xcd, 0x2b,
+    0x11, 0xf7, 0x10, 0xfd, 0x6f, 0x41, 0x92, 0x82, 0x7c, 0x3a, 0x71, 0x07,
+    0x67, 0xd0, 0x7e, 0xb7, 0xdf, 0xdc, 0xfc, 0xee, 0xe6, 0x55, 0xdd, 0x6f,
+    0x79, 0x23, 0xf3, 0xae, 0xb1, 0x21, 0x96, 0xbe, 0xea, 0x0e, 0x1b, 0x58,
+    0x0b, 0x3f, 0x63, 0x51, 0xd4, 0xce, 0x98, 0xfe, 0x1a, 0xc7, 0xa7, 0x43,
+    0x7f, 0x0c, 0xe8, 0x62, 0xcf, 0x78, 0x3f, 0x4e, 0x31, 0xbf, 0x2b, 0x76,
+    0x91, 0xcd, 0x19, 0x80, 0x0d, 0x7f, 0x11, 0x8b, 0x76, 0xef, 0x43, 0x3c,
+    0x4f, 0x61, 0x86, 0xc5, 0x64, 0xa8, 0xc2, 0x73, 0xc2, 0x64, 0x39, 0xa0,
+    0x8b, 0xe6, 0x7f, 0xf6, 0x26, 0xd4, 0x47, 0x4f, 0xe4, 0x46, 0xe2, 0xf5,
+    0x9e, 0xe6, 0xc7, 0x76, 0x6c, 0xa9, 0x0f, 0x1d, 0x1b, 0x22, 0xa5, 0x62,
+    0x0a, 0x88, 0x3e, 0x8c, 0xf0, 0xbc, 0x4c, 0x11, 0x3f, 0x0d, 0xf7, 0x85,
+    0x67, 0x0b, 0x4c, 0xa3, 0x3f, 0xa8, 0xf1, 0x2a, 0x65, 0x2e, 0x00, 0x03,
+    0xc9, 0x49, 0x91, 0x48, 0xb7, 0xc8, 0x29, 0x28, 0x2f, 0x46, 0x8e, 0x8b,
+    0xd6, 0x73, 0x19, 0x06, 0x3e, 0x6f, 0x92, 0xc8, 0x3d, 0x3f, 0x4d, 0x68,
+    0xbc, 0x02, 0xc0, 0x8f, 0x71, 0x46, 0x0d, 0x28, 0x63, 0xfe, 0xad, 0x14,
+    0x81, 0x04, 0xb7, 0x23, 0xfd, 0x21, 0x0a, 0xf0, 0x6f, 0xcd, 0x47, 0x0b,
+    0x0e, 0x93, 0xa3, 0xa8, 0x44, 0x15, 0xd6, 0xae, 0x06, 0x44, 0x6b, 0xbc,
+    0xff, 0x8a, 0x56, 0x60, 0x3c, 0x38, 0xd6, 0xed, 0x03, 0x2d, 0x79, 0x2a,
+    0xe9, 0x15, 0xef, 0xfc, 0x92, 0x1f, 0x83, 0xa4, 0x60, 0x8f, 0xc9, 0x29,
+    0xb2, 0xb4, 0x9e, 0x3f, 0xa9, 0xe8, 0xfb, 0xa2, 0x62, 0x20, 0x2e, 0xc9,
+    0x43, 0xb2, 0xd1, 0x36, 0x85, 0x1e, 0xa4, 0xb3, 0x4f, 0x8c, 0x9e, 0x81,
+    0x75, 0x68, 0xbc, 0xf1, 0x52, 0xd5, 0x03, 0x22, 0xcf, 0xdf, 0x64, 0xb0,
+    0x28, 0xd2, 0x45, 0x18, 0x38, 0x8c, 0xd0, 0xf6, 0x30, 0x3c, 0x04, 0xd9,
+    0x8d, 0xb6, 0xb2, 0x57, 0x2a, 0xee, 0x28, 0xeb, 0x5f, 0x1a, 0x10, 0x6e,
+    0x88, 0x79, 0x08, 0x23, 0x19, 0x84, 0xf8, 0x80, 0x1a, 0x7d, 0x6f, 0x8b,
+    0xc1, 0x8e, 0x5f, 0x5f, 0x54, 0x14, 0x2a, 0xdc, 0x41, 0x5d, 0xeb, 0x00,
+    0xf2, 0x50, 0xae, 0xd3, 0x55, 0x32, 0xf6, 0xd9, 0x34, 0xf4, 0xb2, 0xf2,
+    0xf5, 0x90, 0x05, 0x8a, 0x9c, 0xc7, 0x94, 0x5d, 0x2d, 0x5a, 0x0f, 0xdd,
+    0x03, 0xde, 0xbe, 0x18, 0xb3, 0xe3, 0x07, 0x6b, 0x57, 0xfa, 0x1b, 0x7b,
+    0x75, 0xcb, 0xc2, 0x4d, 0xf7, 0x88, 0xfe, 0xf9, 0xc0, 0x6c, 0xdb, 0x5f,
+    0xf6, 0x48, 0x00, 0x4a, 0x5d, 0x75, 0xfa, 0x6b, 0x45, 0x43, 0xc4, 0x7f,
+    0x97, 0x31, 0x22, 0xb4, 0x9c, 0xa3, 0xee, 0x2f, 0x27, 0xa9, 0x9f, 0x0e,
+    0xdc, 0x40, 0x67, 0x17, 0x2e, 0xcb, 0xfd, 0x9e, 0xe7, 0xb2, 0x85, 0xcd,
+    0x49, 0x24, 0xc8, 0x8a, 0x59, 0x6b, 0x1f, 0xec, 0x72, 0x89, 0xf8, 0x30,
+    0xdf, 0x82, 0x61, 0x3b, 0x8b, 0xc9, 0x80, 0xe4, 0x27, 0x0d, 0xfe, 0x42,
+    0x27, 0x6c, 0xaf, 0x62, 0x3e, 0x2f, 0x1d, 0x38, 0xb6, 0x88, 0x8f, 0x71,
+    0x5a, 0x54, 0x6c, 0x68, 0x57, 0x40, 0x49, 0x7a, 0xb2, 0xe8, 0xb6, 0x97,
+    0xab, 0xd6, 0x3c, 0x35, 0xf3, 0x95, 0x12, 0xde, 0xa2, 0x39, 0x54, 0x52,
+    0x8c, 0x38, 0x2a, 0x2b, 0xe7, 0x21, 0x38, 0x63, 0xb0, 0xd6, 0xad, 0x94,
+    0x44, 0xaf, 0x49, 0x5d, 0xfc, 0x49, 0x6b, 0x30, 0xdf, 0xe9, 0x19, 0x1e,
+    0xed, 0x98, 0x0d, 0x4a, 0x3d, 0x56, 0x5e, 0x74, 0xad, 0x13, 0x8b, 0x68,
+    0x45, 0x08, 0xbe, 0x0e, 0x6c, 0xb4, 0x62, 0x93, 0x27, 0x8b, 0x4f, 0xab,
+    0x3e, 0xba, 0xe1, 0xe5, 0xff, 0xa8, 0x5d, 0x33, 0x32, 0xff, 0x34, 0xf9,
+    0x8d, 0x67, 0x24, 0x4a, 0xbb, 0x2c, 0x60, 0xb5, 0x88, 0x96, 0x1b, 0xcc,
+    0x53, 0xfb, 0x2e, 0x05, 0x1d, 0x8b, 0xc2, 0xa0, 0xde, 0x21, 0x41, 0x5e,
+    0x11, 0x1b, 0x96, 0xd9, 0xa6, 0xae, 0xbd, 0xf0, 0x91, 0xad, 0x69, 0x2b,
+    0xd2, 0x3f, 0xe4, 0x3d, 0x16, 0x69, 0xa6, 0xb2, 0x9c, 0xbe, 0x59, 0x7b,
+    0x87, 0x79, 0xf5, 0xc2, 0x5a, 0xcc, 0xdf, 0xfe, 0x7f, 0xf9, 0xa6, 0x52,
+    0xde, 0x5f, 0x46, 0x91, 0x21, 0x2c, 0x2c, 0x49, 0x25, 0x00, 0xd5, 0xe4,
+    0x81, 0x6b, 0x85, 0xad, 0x98, 0xaf, 0x06, 0x4a, 0x83, 0xb2, 0xe3, 0x42,
+    0x39, 0x31, 0x50, 0xe1, 0x2d, 0x22, 0xe6, 0x07, 0x24, 0x65, 0x29, 0x3f,
+    0x4c, 0xbd, 0x14, 0x8d, 0xfa, 0x31, 0xfa, 0xa4, 0xb5, 0x99, 0x04, 0xa2,
+    0xa5, 0xcc, 0x3b, 0x12, 0xb1, 0xaa, 0x6a, 0x17, 0x78, 0x8b, 0xb3, 0xe4,
+    0x3c, 0x4c, 0xc5, 0xaa, 0x79, 0x12, 0x17, 0xe0, 0x22, 0x4d, 0xf4, 0xa9,
+    0xd5, 0xd0, 0xed, 0xf8, 0xfe, 0x0a, 0x45, 0x80, 0x9f, 0x3b, 0x74, 0xa0,
+    0xb1, 0xda, 0x18, 0xfa, 0xc2, 0x7d, 0xf6, 0x18, 0x2e, 0xa9, 0x2b, 0x7e,
+    0x69, 0x06, 0x43, 0x2d, 0x62, 0x09, 0x42, 0x10, 0x9f, 0x83, 0xad, 0xd9,
+    0xdd, 0xcd, 0xcb, 0x1b, 0x33, 0x32, 0x3e, 0x1f, 0xf6, 0xac, 0x3b, 0xa3,
+    0x29, 0xd7, 0xc0, 0x88, 0xf9, 0xb7, 0x4c, 0xcd, 0x0a, 0x1f, 0xb8, 0x0f,
+    0xe6, 0xf7, 0xd7, 0x4d, 0x5f, 0x06, 0x12, 0x8a, 0x12, 0xa6, 0x2d, 0xbe,
+    0x5c, 0x57, 0xf8, 0x7f, 0x54, 0x3f, 0x90, 0x83, 0x2c, 0x0a, 0xc5, 0x3d,
+    0x03, 0x78, 0x8a, 0x68, 0xf0, 0xbd, 0xa5, 0x3e, 0xe7, 0x07, 0xab, 0xc8,
+    0x58, 0x2f, 0x5c, 0xfd, 0xb5, 0x39, 0xe3, 0xc6, 0x1c, 0x27, 0xf9, 0x0b,
+    0xc7, 0x4c, 0xcc, 0x67, 0x62, 0xe6, 0x79, 0xe8, 0xc1, 0x0a, 0x86, 0x8a,
+    0xb2, 0x32, 0x7b, 0x90, 0x36, 0x50, 0x92, 0x1f, 0x3e, 0x68, 0x39, 0x1c,
+    0x4d, 0x5d, 0xf8, 0x2b, 0xe8, 0x7d, 0xe2, 0x34, 0x61, 0x9e, 0xc3, 0x77,
+    0xb9, 0x4c, 0x34, 0x08, 0xda, 0x31, 0xc9, 0x1d, 0xbd, 0x3b, 0x7b, 0xf1,
+    0x14, 0xba, 0x3a, 0x34, 0x13, 0xaa, 0x5e, 0xa8, 0x36, 0xf6, 0xfe, 0xed,
+    0x5b, 0xef, 0xaf, 0x24, 0x42, 0xba, 0xfc, 0xc9, 0x30, 0x84, 0xec, 0x49,
+    0x14, 0xab, 0x58, 0x71, 0xfe, 0x4b, 0x6d, 0x7b, 0x9f, 0xbb, 0x3c, 0x83,
+    0xdf, 0x3a, 0xfb, 0x54, 0xff, 0x36, 0xaa, 0x6c, 0x47, 0x94, 0xc0, 0xde,
+    0x89, 0x2e, 0xac, 0x68, 0xee, 0xe8, 0xf4, 0xae, 0xa3, 0xe0, 0x91, 0x55,
+    0x0b, 0x0c, 0xd7, 0xf4, 0x33, 0xb5, 0xf9, 0xf2, 0x9e, 0xda, 0x78, 0xe5,
+    0x75, 0xec, 0xdb, 0xf6, 0xed, 0x27, 0x9f, 0x44, 0x19, 0x9f, 0xb7, 0xf0,
+    0xac, 0x1b, 0x3a, 0xf5, 0x77, 0xc7, 0x76, 0x1e, 0x3f, 0x78, 0x12, 0x48,
+    0x1d, 0xb8, 0xe0, 0x30, 0x29, 0x9a, 0x8c, 0x8f, 0x21, 0x44, 0x9c, 0x89,
+    0xec, 0x8e, 0xd0, 0x81, 0xf5, 0x6a, 0xd0, 0xac, 0x5e, 0xf0, 0x0f, 0x88,
+    0x86, 0x31, 0x2e, 0x15, 0x1e, 0x0d, 0x2d, 0xeb, 0x56, 0x30, 0x27, 0x02,
+    0x93, 0xf4, 0x07, 0x07, 0xba, 0xf7, 0xbd, 0xe8, 0x27, 0x4f, 0xc6, 0xd9,
+    0x57, 0x10, 0x3b, 0xf0, 0xff, 0x2f, 0x2d, 0x6b, 0xd0, 0x17, 0xb3, 0x49,
+    0xeb, 0xc2, 0x49, 0xdb,
+};
+
+
 static_assert(sizeof(kInput) == sizeof(kOutput),
               "Input and output lengths don't match.");
+static_assert(sizeof(kInput) == sizeof(kOverflowOutput),
+              "Input and output lengths don't match.");
 
 TEST(ChaChaTest, TestVector) {
   // Run the test with the test vector at all lengths.
@@ -237,6 +331,22 @@ TEST(ChaChaTest, TestVector) {
   }
 }
 
+TEST(ChaChaTest, CounterOverflow) {
+  // Run the test with the test vector at all lengths.
+  for (size_t len = 0; len <= sizeof(kInput); len++) {
+    SCOPED_TRACE(len);
+
+    std::unique_ptr<uint8_t[]> buf(new uint8_t[len]);
+    CRYPTO_chacha_20(buf.get(), kInput, len, kKey, kNonce, kOverflowCounter);
+    EXPECT_EQ(Bytes(kOverflowOutput, len), Bytes(buf.get(), len));
+
+    // Test the in-place version.
+    OPENSSL_memcpy(buf.get(), kInput, len);
+    CRYPTO_chacha_20(buf.get(), buf.get(), len, kKey, kNonce, kOverflowCounter);
+    EXPECT_EQ(Bytes(kOverflowOutput, len), Bytes(buf.get(), len));
+  }
+}
+
 #if defined(CHACHA20_ASM) && defined(SUPPORTS_ABI_TEST)
 TEST(ChaChaTest, ABI) {
   uint32_t key[8];
diff --git a/crypto/chacha/internal.h b/crypto/chacha/internal.h
index 1435e3b01e6..5f442ec4616 100644
--- a/crypto/chacha/internal.h
+++ b/crypto/chacha/internal.h
@@ -32,7 +32,14 @@ void CRYPTO_hchacha20(uint8_t out[32], const uint8_t key[32],
      defined(OPENSSL_ARM) || defined(OPENSSL_AARCH64))
 #define CHACHA20_ASM
 
-// ChaCha20_ctr32 is defined in asm/chacha-*.pl.
+// ChaCha20_ctr32 encrypts |in_len| bytes from |in| and writes the result to
+// |out|. If |in| and |out| alias, they must be equal.
+//
+// |counter[0]| is the initial 32-bit block counter, and the remainder is the
+// 96-bit nonce. If the counter overflows, the output is undefined. The function
+// will produce output, but the output may vary by machine and may not be
+// self-consistent. (On some architectures, the assembly implements a mix of
+// 64-bit and 32-bit counters.)
 void ChaCha20_ctr32(uint8_t *out, const uint8_t *in, size_t in_len,
                     const uint32_t key[8], const uint32_t counter[4]);
 #endif
diff --git a/include/openssl/chacha.h b/include/openssl/chacha.h
index cfbaa75680e..2868c290626 100644
--- a/include/openssl/chacha.h
+++ b/include/openssl/chacha.h
@@ -29,6 +29,12 @@ extern "C" {
 // CRYPTO_chacha_20 encrypts |in_len| bytes from |in| with the given key and
 // nonce and writes the result to |out|. If |in| and |out| alias, they must be
 // equal. The initial block counter is specified by |counter|.
+//
+// This function implements a 32-bit block counter as in RFC 8439. On overflow,
+// the counter wraps. Reusing a key, nonce, and block counter combination is not
+// secure, so wrapping is usually a bug in the caller. While it is possible to
+// wrap without reuse with a large initial block counter, this is not
+// recommended and may not be portable to other ChaCha20 implementations.
 OPENSSL_EXPORT void CRYPTO_chacha_20(uint8_t *out, const uint8_t *in,
                                      size_t in_len, const uint8_t key[32],
                                      const uint8_t nonce[12], uint32_t counter);