diff --git a/docs/development/test-vectors.rst b/docs/development/test-vectors.rst
index 6a7a35e..207515b 100644
--- a/docs/development/test-vectors.rst
+++ b/docs/development/test-vectors.rst
@@ -121,6 +121,9 @@
 * ``unsupported_extension.pem`` - An RSA 2048 bit self-signed certificate
   containing an unsupported extension type. The OID was encoded as
   "1.2.3.4" with an ``extnValue`` of "value".
+* ``unsupported_extension_critical.pem`` - An RSA 2048 bit self-signed
+  certificate containing an unsupported extension type marked critical. The OID
+  was encoded as "1.2.3.4" with an ``extnValue`` of "value".
 
 Custom X.509 Request Vectors
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/vectors/cryptography_vectors/x509/custom/unsupported_extension_critical.pem b/vectors/cryptography_vectors/x509/custom/unsupported_extension_critical.pem
new file mode 100644
index 0000000..7c1473e
--- /dev/null
+++ b/vectors/cryptography_vectors/x509/custom/unsupported_extension_critical.pem
@@ -0,0 +1,18 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----