Koepka, who displaced Dustin Johnson for the world's top ranking, shot a final-round 8-under 64 for a 21-under total of 267. Koepka, who had never held the top ranking, led by four strokes after three rounds.
"To be No. 1 is something I've dreamed of as a kid," Koepka said. "I don't think this one is going to sink in."