Haas shared the lead after 54 holes with three competitors but put some daylight between himself and the pack Sunday thanks to three straight birdies beginning with the eighth hole at Congressional Country Club.
His performance netted him a 12-under-par 272, good enough for a fifth career PGA Tour win.
Settling for sole possession of second place at 9-under 275 was Roberto Castro, another third-round co-leader, who carded a 2-under 69.
Knotted for third at minus-8 were D.H. Lee, who charged up the leaderboard with a 7-under 64, and Jason Kokrak (69).
Stewart Cink (67) claimed fourth one shot further back while 19-year-old Jordan Spieth (69), who held a share of the lead after the second round, took fifth at minus-6.