Taylor Swift got an unwanted groom on her Rhode Island doorstep ... and no, we don't mean Calvin Harris.

According to cops, the man drove to her Westerly, RI mansion Wednesday ... claiming he was scheduled to see Taylor. Witnesses tell TMZ the man was crying and told officers Taylor was supposed to marry him, but now she's with Tom.

Taylor's security team first stopped him at her gate and called police, who transported him to a hospital for mental evaluation. His car was towed from the scene.

It's unclear if Swift was home at the time of the incident. She hopped a flight to Australia late on Wednesday.