diff --git a/jose/backends/cryptography_backend.py b/jose/backends/cryptography_backend.py
index abd24260..f636a39c 100644
--- a/jose/backends/cryptography_backend.py
+++ b/jose/backends/cryptography_backend.py
@@ -243,8 +243,8 @@ def __init__(self, key, algorithm, cryptography_backend=default_backend):
 
         self.cryptography_backend = cryptography_backend
 
-        # if it conforms to RSAPublicKey interface
-        if hasattr(key, "public_bytes") and hasattr(key, "public_numbers"):
+        # if it conforms to RSAPublicKey or RSAPrivateKey interface
+        if (hasattr(key, "public_bytes") and hasattr(key, "public_numbers")) or hasattr(key, "private_bytes"):
             self.prepared_key = key
             return