Kaymer's closing double handed Ilonen his second win of the season. Ilonen completed the event at 6-under-par 274 thanks to a final-round 68.
'Unbelievable finish. First I dumped it in the water on 16, then I miss a birdie on 17,' Ilonen stated. 'Then for the first time, I hit that green on 18 and make birdie. I'm out of words.'
The 22-year-old Kaymer, who was going for his first win, shot 3-over 73 and ended in a share of second at minus-4. He was joined there by Peter Hedblom (69), Jean-Baptiste Gonnet (68), Christian Cevaer (69) and Nick Dougherty (70).
Kaymer and Ilonen, who hails from Finland, battled throughout the round. After both players birdied the par-5 15th, it looked as though the 16th at Arlandastad Golf Club would be the turning point.
Ilonen's tee ball came to rest on the edge of the water right of the green. He tried to play out of the water, but advanced his ball just a few feet. He pitched his third to 8 feet, but two-putted for double-bogey to slide to 5 under.
Kaymer, now leading by two, knocked his tee shot over the green at the 16th. He chipped to 7 feet, but only two-putted for bogey to slip to 6 under.
Neither player could take advantage of the par-5 17th as they both made par.
Ilonen, playing one group ahead of Kaymer, hit a stellar tee shot to 6 feet at the 18th. He knocked that in for birdie to move to minus-6, then watched from the scoring trailer to see what Kaymer would do.
Kaymer pulled his tee shot left, then his chip came up short of the green. He tried to putt through the fringe, but his ball stopped 8 feet short of the hole. He two-putted from there for double-bogey to end tied for second.
'You never wish for a finish like that, but someone has to win and someone has to lose,' said Ilonen. 'I'm sure Martin is going to win very soon. He's been up there a couple of times now. I'm sure he's lifting a trophy in the next few months.'
Ilonen collected his second win of the season as he also won the Indonesia Open back in February.
Ilonen had three birdies and a bogey over the first six holes to move to minus-6, one clear of Kaymer who bogeyed four and five. Ilonen bogeyed the seventh, but recovered that shot with a birdie on nine.
Kaymer birdied the ninth to join Ilonen in the lead at 6 under and set up the big finish.
Cevaer posted four rounds of 1-under 69 to end in a share of second. He posted four birdies and three bogeys on Sunday. Dougherty managed just two birdies and two bogeys in the final round to end at minus-4.
Gonnet parred his first 11 holes, before collecting two birdies over the final seven holes to cap a bogey-free 68 that left him tied for second.
Hedblom opened with nine straight pars before collecting three birdies and two bogeys on the back nine to end at 4 under.
James Kingston, who entered the final round tied for the lead with Kaymer, struggled to a 4-over 74. That dropped him into a share of seventh at 3-under-par 277, where he was joined by Paul Broadhurst and Corey Pavin.