Philip Barton Key (April 5, 1818 – February 27, 1859) was a United States Attorney for the District of Columbia. He is most famous for his public affair with Teresa Bagioli Sickles, and his eventual murder at the hands of her husband, Congressman Daniel Sickles of New York. Sickles defended himself by adopting a defense of temporary insanity, the first time the defense had been used in the United States.