diff --git a/handshake_client.go b/handshake_client.go
index 3aa4ef3..9f766ee 100644
--- a/handshake_client.go
+++ b/handshake_client.go
@@ -1162,9 +1162,14 @@ func (c *Conn) verifyServerCertificate(certificates [][]byte) error {
 		}
 	}
 
+	if fips140tls.Required() && !isCertificateAllowedFIPS(certs[0]) {
+		c.sendAlert(alertBadCertificate)
+		err := errors.New("server's certificate is not allowed in FIPS 140-3 mode")
+		return &CertificateVerificationError{UnverifiedCertificates: certs, Err: err}
+	}
+
 	switch certs[0].PublicKey.(type) {
 	case *rsa.PublicKey, *ecdsa.PublicKey, ed25519.PublicKey:
-		break
 	case *mldsa.PublicKey:
 		if c.vers < VersionTLS13 {
 			c.sendAlert(alertIllegalParameter)
diff --git a/handshake_server.go b/handshake_server.go
index 4e8d531..a749cc0 100644
--- a/handshake_server.go
+++ b/handshake_server.go
@@ -994,6 +994,12 @@ func (c *Conn) processCertsFromClient(certificate Certificate) error {
 	c.scts = certificate.SignedCertificateTimestamps
 
 	if len(certs) > 0 {
+		if fips140tls.Required() && !isCertificateAllowedFIPS(certs[0]) {
+			c.sendAlert(alertBadCertificate)
+			err := errors.New("client's certificate is not allowed in FIPS 140-3 mode")
+			return &CertificateVerificationError{UnverifiedCertificates: certs, Err: err}
+		}
+		
 		switch certs[0].PublicKey.(type) {
 		case *ecdsa.PublicKey, *rsa.PublicKey, ed25519.PublicKey:
 		case *mldsa.PublicKey: