Castro bogeyed the opening hole, and that was his only mistake. He was one shot out of the lead at the turn, couldn't match birdies with Haas at the par-3 10th, and then stuck with him the rest of the day.
"It helped that Roberto played so well," Haas said.
Haas, who finished on 12-under 272, never allowed himself to think about winning, even after he seized control around the turn. Congressional wouldn't let him. Even though he made 15 birdies on the weekend, he remembered the triple bogey on the 11th hole Saturday that temporarily derailed him.
This time, he found the fairway, hit onto the green, took two putts for par and exhaled.
Haas saved par from a bunker on the par-3 13th with a 6-foot putt that swirled 360 degrees around the cup before falling, and then picked up an unlikely birdie on the 14th when his 9-iron was drifting toward a mound covered with shaggy rough to the right of the green. It hopped off the mound to about 10 feet, and he went from a possible bogey to a birdie when he made the putt.
He made one more birdie with a wedge that checked up a foot from hole on the par-5 16th, and Haas was on his way.
The biggest struggle after that was hoisting the silver trophy of the U.S. Capitol over his head in the stifling heat of the closing ceremony on the 18th green.
Haas was still smarting over losing a three-shot lead in the final round at Riviera, making five bogeys in a seven-hole stretch in the middle of his round. He had the 36-hole lead at the Memorial until a 76-71 weekend.
He was solid on Sunday at Congressional, and the win moved him to No. 7 in the FedEx Cup standings with the playoffs about two months away. That's important to Haas, who won the FedEx Cup in 2011 and failed to qualify for the Tour Championship last year.
D.H. Lee made nine birdies to match a tournament-best 64 and tied for third with Jason Kokrak, who briefly shared the lead on the front nine and had a 69. Stewart Cink closed with a 67 and finished alone in fifth, his best finish on the PGA Tour in stroke play since he won the British Open four years ago at Turnberry.