Lawler hadn't competed in the UFC since 2004, and his opponent Koscheck, a former UFC welterweight top contender, was near a 4-to-1 favorite.
Koscheck controlled the bout early with his wrestling but the fight's complexion changed when Lawler got back to his feet and landed a knee to the face. He swarmed Koscheck with a barrage of rights. Koscheck went down and Lawler continued the assault until referee Herb Dean stepped in with 3:57 left in the first round.
Koscheck immediately protested the stoppage, but to no avail.
"I was raining down hard shots and he was going to take a beating," Lawler said. "He wasn't coming back."
Lawler is now 20-9 with 1 no contest.
Koscheck, who hadn't fought since last May, has now dropped two in a row, falling to 17-7.