diff --git a/doc/godebug.md b/doc/godebug.md
index 41f88caa01..86e02e820c 100644
--- a/doc/godebug.md
+++ b/doc/godebug.md
@@ -185,11 +185,6 @@ Go 1.23 changed the behavior of
 serial numbers that are negative. This change can be reverted with
 the [`x509negativeserial` setting](/pkg/crypto/x509/#ParseCertificate).
 
-Go 1.23 changed the behavior of
-[crypto/x509.ParseCertificate](/pkg/crypto/x509/#ParseCertificate) to reject
-serial numbers that are longer than 20 octets. This change can be reverted with
-the [`x509seriallength` setting](/pkg/crypto/x509/#ParseCertificate).
-
 Go 1.23 re-enabled support in html/template for ECMAScript 6 template literals by default.
 The [`jstmpllitinterp` setting](/pkg/html/template#hdr-Security_Model) no longer has
 any effect.
diff --git a/src/crypto/x509/parser.go b/src/crypto/x509/parser.go
index 5cc0c7742e..3ba5f6a4e1 100644
--- a/src/crypto/x509/parser.go
+++ b/src/crypto/x509/parser.go
@@ -825,7 +825,6 @@ func processExtensions(out *Certificate) error {
 }
 
 var x509negativeserial = godebug.New("x509negativeserial")
-var x509seriallength = godebug.New("x509seriallength")
 
 func parseCertificate(der []byte) (*Certificate, error) {
 	cert := &Certificate{}
@@ -866,27 +865,10 @@ func parseCertificate(der []byte) (*Certificate, error) {
 		return nil, errors.New("x509: invalid version")
 	}
 
-	var serialBytes cryptobyte.String
-	if !tbs.ReadASN1Element(&serialBytes, cryptobyte_asn1.INTEGER) {
-		return nil, errors.New("x509: malformed serial number")
-	}
-	// We add two bytes for the tag and length (if the length was multi-byte,
-	// which is possible, the length of the serial would be more than 256 bytes,
-	// so this condition would trigger anyway).
-	if len(serialBytes) > 20+2 {
-		if x509seriallength.Value() != "1" {
-			return nil, errors.New("x509: serial number too long (>20 octets)")
-		} else {
-			x509seriallength.IncNonDefault()
-		}
-	}
 	serial := new(big.Int)
-	if !serialBytes.ReadASN1Integer(serial) {
+	if !tbs.ReadASN1Integer(serial) {
 		return nil, errors.New("x509: malformed serial number")
 	}
-	// We do not reject zero serials, because they are unfortunately common
-	// in important root certificates which will not expire for a number of
-	// years.
 	if serial.Sign() == -1 {
 		if x509negativeserial.Value() != "1" {
 			return nil, errors.New("x509: negative serial number")
@@ -1034,10 +1016,6 @@ func parseCertificate(der []byte) (*Certificate, error) {
 // Before Go 1.23, ParseCertificate accepted certificates with negative serial
 // numbers. This behavior can be restored by including "x509negativeserial=1" in
 // the GODEBUG environment variable.
-//
-// Before Go 1.23, ParseCertificate accepted certificates with serial numbers
-// longer than 20 octets. This behavior can be restored by including
-// "x509seriallength=1" in the GODEBUG environment variable.
 func ParseCertificate(der []byte) (*Certificate, error) {
 	cert, err := parseCertificate(der)
 	if err != nil {
diff --git a/src/crypto/x509/x509_test.go b/src/crypto/x509/x509_test.go
index d40fd836e0..a9483b7091 100644
--- a/src/crypto/x509/x509_test.go
+++ b/src/crypto/x509/x509_test.go
@@ -4086,26 +4086,3 @@ func TestRejectCriticalSKI(t *testing.T) {
 		t.Fatalf("ParseCertificate() unexpected error: %v, want: %s", err, expectedErr)
 	}
 }
-
-func TestSerialTooLong(t *testing.T) {
-	template := Certificate{
-		Subject:   pkix.Name{CommonName: "Cert"},
-		NotBefore: time.Unix(1000, 0),
-		NotAfter:  time.Unix(100000, 0),
-	}
-	for _, serial := range []*big.Int{
-		big.NewInt(0).SetBytes(bytes.Repeat([]byte{5}, 21)),
-		big.NewInt(0).SetBytes(bytes.Repeat([]byte{255}, 20)),
-	} {
-		template.SerialNumber = serial
-		certDER, err := CreateCertificate(rand.Reader, &template, &template, rsaPrivateKey.Public(), rsaPrivateKey)
-		if err != nil {
-			t.Fatalf("CreateCertificate() unexpected error: %v", err)
-		}
-		expectedErr := "x509: serial number too long (>20 octets)"
-		_, err = ParseCertificate(certDER)
-		if err == nil || err.Error() != expectedErr {
-			t.Fatalf("ParseCertificate() unexpected error: %v, want: %s", err, expectedErr)
-		}
-	}
-}
diff --git a/src/internal/godebugs/table.go b/src/internal/godebugs/table.go
index b44fc7874f..eb51255916 100644
--- a/src/internal/godebugs/table.go
+++ b/src/internal/godebugs/table.go
@@ -57,7 +57,6 @@ var All = []Info{
 	{Name: "winsymlink", Package: "os", Changed: 22, Old: "0"},
 	{Name: "x509keypairleaf", Package: "crypto/tls", Changed: 23, Old: "0"},
 	{Name: "x509negativeserial", Package: "crypto/x509", Changed: 23, Old: "1"},
-	{Name: "x509seriallength", Package: "crypto/x509", Changed: 23, Old: "1"},
 	{Name: "x509sha1", Package: "crypto/x509"},
 	{Name: "x509usefallbackroots", Package: "crypto/x509"},
 	{Name: "x509usepolicies", Package: "crypto/x509"},
diff --git a/src/runtime/metrics/doc.go b/src/runtime/metrics/doc.go
index 85db5742d9..c1d0ca9072 100644
--- a/src/runtime/metrics/doc.go
+++ b/src/runtime/metrics/doc.go
@@ -340,11 +340,6 @@ Below is the full list of supported metrics, ordered lexicographically.
 		package due to a non-default GODEBUG=x509negativeserial=...
 		setting.
 
-	/godebug/non-default-behavior/x509seriallength:events
-		The number of non-default behaviors executed by the crypto/x509
-		package due to a non-default GODEBUG=x509seriallength=...
-		setting.
-
 	/godebug/non-default-behavior/x509sha1:events
 		The number of non-default behaviors executed by the crypto/x509
 		package due to a non-default GODEBUG=x509sha1=... setting.