Davis used his physicality to dominate the unbeaten Martin before unloading against the challenger in the eighth round.
After backing Martin onto the ropes with a flurry of punches, Davis connected with a devastating left hook that sent the challenger crashing to the canvas at the MGM Grand Garden Arena.
Davis improved to 30-0, with 28 knockouts following the win against Martin, the 29-year-old from Detroit who had been unbeaten in 18 fights before Saturday's defeat.