Flight 90 of the North American X-15 was a research flight conducted by NASA and the US Air Force in 1963. It was the first of two X-15 missions that passed the 100-km high Krmn line, the FAI definition of space, along with Flight 91 the next month. The X-15 was flown by Joseph A. Walker, who flew both X-15 spaceflights over the Krmn line.

Joseph Albert Walker (February 20, 1921 – June 8, 1966) (Capt, USAF) was an American World War II pilot, experimental physicist, NASA test pilot, and astronaut. He was one of twelve pilots who flew the North American X-15, an experimental spaceplane jointly operated by the Air Force and NASA.

In 1963 Walker made three flights above 50 miles, thereby qualifying as an astronaut according to the United States definition of the boundary of space. The latter two, X-15 Flights 90 and 91, also surpassed the Kármán line, the internationally accepted boundary of 100 kilometers (62.14 miles). Making the latter flights immediately after the completion of the Mercury and Vostok programs, Walker became the first person to fly to space twice. He was the only X-15 pilot to fly above 100 km during the program.