The Northrop YF-23 or Northrop-McDonnell Douglas YF-23 is an American single-seat, twin-engine fighter aircraft designed for the United States Air Force (USAF). The design was a finalist in the USAF's Advanced Tactical Fighter (ATF) competition, beating out the Lockheed YF-22 for the production contract. Two YF-23 prototypes were built, nicknamed "Black Widow II" and "Gray Ghost".In the 1980s the USAF began looking for a replacement for its fighter aircraft, primarily to counter the Soviet Union's advanced Su-27 and MiG-29 aircraft. Several companies submitted design proposals, the USAF's chosen proposals were from Northrop and Lockheed . Northrop worked with McDonnell Douglas to develop the YF-23, while Lockheed, Boeing and General Dynamics developed the YF-22.

The YF-23 had the advantages of being stealthier and faster, but less agile than the competition. After a four-year development and evaluation process, the YF-22 was declared the winner in 1991 and entered production as the Lockheed Martin F-22 Raptor. The US Navy was interested in using the production version of the ATF as the basis for a replacement for the F-14 , but this plan was later canceled. As of 2009, two YF-23 prototypes are on museum display.