Pacific Palisades, CA (SportsNetwork.com) - James Hahn birdied the third playoff hole on Sunday to win the Northern Trust Open.

Hahn, Dustin Johnson and Paul Casey all finished at 6-under-par 278. Hahn and Johnson, who played together in the final round, carded 2-under 69s, while Casey shot 3-under 68 to get into the playoff.

"This is amazing. This is unbelievable. I never would have thought I would have won this golf tournament, there's so many great players here," Hahn said. "Just to be here competing with these guys is an honor."

The group returned to the 18th at Riviera Country Club for the first playoff hole. Hahn and Johnson found the green with their approach shots, while Casey came up short with his second. After Casey chipped to three feet, Hahn and Johnson 2-putted for their pars. Casey kicked in his par and they were off to the short, but difficult par-4 10th.

Johnson and Hahn found the left rough off the tee and Casey's tee ball landed in the short grass. Casey chipped to about 15 feet. Hahn and Johnson both hit stellar chip shots, Hahn to 12 feet and Johnson to four feet.

Casey's birdie effort slid by the hole and he marked. Hahn drained his birdie putt, and Johnson dropped his birdie try in as well. Casey was eliminated, while Hahn and Johnson moved to the par-3 14th.

Johnson dropped his tee shot 11 feet from the hole. Hahn followed by hitting his tee ball to 17 feet. Hahn poured in his birdie putt.

"I was telling my caddie, if I could last at least one playoff hole, I think I have a good chance to kind of get the nerves calmed down," Hahn stated. "Hole 10 was a good birdie and this one (at 14) was a good birdie too. Luckily, I had that same putt in regulation, so I knew which way it broke. I was pretty fortunate for that."

Johnson's birdie effort missed on the left side to give Hahn his first PGA Tour victory.

"He was rolling the putter well all day. He made a lot of key putts and it was no surprise to see that one go in," Johnson said of Hahn. "It was a great tournament, had a lot of fun, but obviously I would have liked to make that last putt to keep it going."

Sergio Garcia bogeyed the final two holes to miss the playoff by a stroke. He posted an even-par 71 and shared fourth at minus-5. Keegan Bradley (68), Hideki Matsuyama (67) and Jordan Spieth (70) ended alongside Garcia. Spieth bogeyed the last as well to miss out on the extra session.

Retief Goosen, who had at least a piece of the lead after the first three rounds, birdied the final two holes for a 4-over 75. He dropped into a tie for eighth at 4-under-par 280. He was joined there by Graham DeLaet (73), Kyle Reifers (67) and Sang-Moon Bae (72).

Seven different players had a piece of the lead during the final round. Hahn had three birdies in the first eight holes to move to minus-7. Hahn parred the ninth and was tied for the lead after Garcia stumbled to a bogey on No. 8.

Hahn chipped his third shot to four feet on the par-5 11th. He knocked in that birdie try to move into the lead by himself. However, Hahn made a mess of the 12th and that led to a bogey.

That bogey dropped Hahn into a 5-way tie for the lead with Bae, Vijay Singh, Garcia and Goosen. After three pars in a row, Hahn found sand off the tee at the par-3 16th. He chipped to seven feet, but 2-putted for bogey to dip to minus-6.

Hahn was one behind Garcia and Johnson at that point, but parred the final two holes to get into the playoff.

Johnson opened with a birdie at the first, then ran off nine straight pars from the second. He got up and down for birdie at the 11th to move to minus-6. Johnson drained a 19-foot birdie try at the 15th to gain a share of the lead.

The eight-time PGA Tour winner found the rough with his tee shot at the par-5 17th. After pitching down the fairway, Johnson dumped his third into a greenside bunker. That led to a bogey, which dropped him to 6-under. He missed a 9-foot birdie putt at 18 that would have given him the title in regulation.

Casey was the first player in the clubhouse at 6-under. He birdied the first and ninth to get to minus-5. He picked up another birdie at the 11th. The Englishman carded back-to-back birdies at 16 and 17, but bogeys at 15 and 18 left him at minus-6.

NOTES: Hahn collected $1.206 million for the victory ... With Goosen failing to win, Brandt Snedeker remains the only 36-hole lead to hang on to win this season on the PGA Tour ... Last year's winner Bubba Watson tripped to a 2-over 73 and fell into a share of 14th at 2-under-par 282 ... The PGA Tour kicks off the Florida swing next week with The Honda Classic, where Russell Henley will defend his title against a field that includes world No. 1 Rory McIlroy.