Penis captivus is an urban legend describing an event that allegedly happens in rare instances during heterosexual intercourse when the muscles in the vagina clamp down on the penis much more firmly than usual, making it impossible for the penis to withdraw from the vagina regardless of erection status.

The first report of the penis captivus sexual phenomenon can be found in an 1884 article by the fictitious Egerton Yorrick Davis in The Philadelphia Medical News. The article was later discovered to be a hoax perpetrated by Sir William Osler. Historians speculate that he was annoyed by an editorial published in the same journal by Dr. Theophilus Parvin, "An Uncommon Form of Vaginismus." Both men served on that respected journal's editorial board.

There is only one known report of penis captivus, in a letter to the British Medical Journal relating to an apparent case in 1947. According to the BMJ, this condition was otherwise unknown in the twentieth century.

Penis captivus should not be confused with the relatively common condition of vaginismus.