| Mission type | Test flight |
|---|---|
| Operator | US Air Force/NASA |
| Mission duration | 11 minutes, 8.6 seconds |
| Distance travelled | 543.4 kilometers (337.7 mi) |
| Apogee | 107.96 kilometers (67.08 mi) |
| Spacecraft properties | |
| Spacecraft | X-15 |
| Manufacturer | North American |
| Launch mass | 15,195 kilograms (33,499 lb) |
| Landing mass | 6,260 kilograms (13,800 lb) |
| Dry mass | 6,577 kilograms (14,500 lb) |
| Crew | |
| Crew size | 1 |
| Members | Joseph A. Walker |
| Start of mission | |
| Launch date | August 22, 1963, 18:05:57 UTC |
| Launch site | NB-52A #52-003, Edwards Dropped over Smith Ranch Dry Lake 39°20′N 117°29′W / 39.333°N 117.483°W |
| End of mission | |
| Landing date | August 22, 1963, 18:17:05 UTC |
| Landing site | Rogers Dry Lake, Edwards |
 Joe Walker | |
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.
Flight 91 was the first internationally recognized spaceflight of a reused spacecraft, as Walker had also flown plane number three on the previous sub-orbital spaceflight over the Kármán line on July 19. The flight was air-launched from a modified Boeing B-52 Stratofortress support plane over Smith Ranch Dry Lake, Nevada, United States. Walker piloted the X-15 to an altitude of 107.96 km and remained weightless for approximately five minutes. The altitude was the highest crewed flight by a spaceplane to that time, and remained the record until the 1981 flight of Space Shuttle Columbia.
Walker landed the X-15 about 12 minutes after it was launched, at Rogers Dry Lake, Edwards Airforce Base, in California. This was Walker's final X-15 flight.