Immelman finished four rounds at 13-under-par 271 for a two-shot victory over Tiger Woods and Mathew Goggin, earning some personal redemption for letting nerves get the best of him earlier this season.
Sunday, he rolled in a 30-foot birdie putt at the 18th hole to finish off his final round, leaving Vijay Singh and Goggin needing hole-outs from the 18th fairway to force a playoff.
The only collapse this time was Immelman's as he fell to his knees.
'It was such a fantastic feeling to hole that putt,' said Immelman, who collected $900,000 for the win. 'Just a thrill really.'
Woods, meanwhile, tied for second place in just his second start since the Masters and first since missing the U.S. Open cut.
Beginning the round three shots behind third-round leader Singh, Woods shot a 3-under 68 to get to 11-under-par 273. He flirted with the lead -- making four birdies in his last eight holes -- but couldn't pull even with Immelman.
Woods' round got off to a bad start when he knocked his opening tee shot into a fairway bunker on the way to a bogey at the par-4 first. He would play the front nine at 1 over before going 4 under on the back.
A 10-foot birdie putt at the 14th, a 15-foot par save at the 15th and a 23-foot birdie putt at the 16th gave him hope, but Woods parred the final two holes for his first runner-up finish since last year.
His stellar back nine wasn't good enough to overcome lapses on the front at Cog Hill.
'Welcome to golf,' Woods joked. 'For some reason, once the flood gates open and once you make one putt [the cup] get a little bigger and they all go in.'
Goggin joined Woods at minus-11 after a round of 2-under 69. He shared the lead with Immelman after 11 holes, but faded after a three-putt bogey at the 12th.
Singh knocked his approach shot at the 18th into the water and finished with a double bogey. He shot a 2-over 73 and shared fourth place with five other players at 9-under-par 275.
Jim Furyk, the 2005 champion, shot a 1-under 70 and joined Singh at minus-9, along with Tim Clark (66), Stephen Leaney (68), Stewart Cink (71) and Carl Pettersson (71).
Leaney and Cink were among those players who challenged Immelman for the lead, but no one could overtake the South African.
Immelman rolled in a 10-foot birdie putt at the 10th hole to take a one-shot lead over three players. At the 15th, he made a 3-footer for birdie to move two shots ahead -- an advantage he maintained with a 10-foot birdie putt at 16.
Then, after stumbling to a bogey at the 17th, Immelman found the bottom of the cup on a putt he figured had at least 6 feet of break to it. The ending was a far cry from two bad finishes earlier this season.
At the Wachovia Championship, Immelman three-putted the final green for bogey and then lost on the first playoff hole to Jim Furyk. The very next week at the Byron Nelson Championship, he pulled his drive at 18 into a lake and finished runner-up to Brett Wetterich.
On Sunday, Immelman, who owns multiple titles on the European Tour, was nearly speechless after wrapping up his first PGA TOUR victory.
'I don't think words can express it,' he said.