X-15 Flight 91 reaches the highest altitude of the X-15 program (107.96 km (67.08 mi) (354,200 feet)).

X-15 Flight 91 was an August 22, 1963 American crewed sub-orbital spaceflight, and the second and final flight in the program to fly above the Kármán line, which was previously achieved during Flight 90 a month earlier by the same pilot, Joseph A. Walker. It was the highest flight of the X-15 program.