The Kh-31 (Russian: Х-31; AS-17 'Krypton') is a Soviet and Russian air-to-surface missile carried by aircraft such as the MiG-29, Su-35 and the Su-57. It is capable of Mach 3.5 and was the first supersonic anti-ship missile that could be launched by tactical aircraft.

In many respects the Kh-31 is a miniaturised version of the P-270 Moskit (SS-N-22 'Sunburn') and was reportedly designed by the same man. The missile is conventionally shaped, with cruciform wings and control surfaces made from titanium. The two-stage propulsion is notable. On launch, a solid-fuel booster in the tail accelerates the missile to Mach 1.8 and the motor is discarded. Then four air intakes open up and, as in the Franco-German ANS/ANF, the empty rocket case becomes the combustion chamber of a kerosene-fuelled ramjet, which takes it beyond Mach 4.

The L-111E seeker of the anti-radar version has a unique antenna, an interferometer array of seven spiral antennas on a steerable platform. The seekers delivered to China in 2001-2 were 106.5 cm (41.9 in) long, 36 cm (14 in) in diameter, and weighed 23 kg (51 lb)