ITALY, 20 Sept 2015, (AFP) - The 28-year-old from Gothenburg drained a 15ft birdie putt at the second play-off hole after a brilliant approach shot from the left rough set up the winning opportunity.
Kaymer was also well positioned but left his 20-footer short of the cup and failed in his bid for a first victory since winning the 2014 US Open.
Both playes had finished 72 holes on 19-under at the par-72 Golf Club Milano on a clear sunny day in Milan.
"It's crazy, I told my caddy on the last five holes that we probably needed five birdies to have a chance and two was enough to get in a play-off and get my first win and to do it against (Martin) Kaymer, it feels unbelievable," said Karlberg.
"I just tried to hit the fairway which I wasn't even close to doing and just seeing him splitting the fairway felt, 'okay, I'm not going to win on his mistake, I need to do a birdie.
"I had a perfect yardage on the last shot there and a good lie, thankfully. It was a little bit lucky but I hit it great. It felt nice."
Seven players tied for third place including Swedes Joakim Lagergren and Jens Fahrbring, English pair Matthew Fitzpatrick and Danny Willett, Paraguayan Fabrizio Zanotti, Lucas Bjerregaard of Denmark and American David Lipsky.
Kaymer had earlier carved out a two-stroke lead, thanks to four birdies down the front nine but three bogeys at 10, 13 and 14 opened the door for Karlberg who pulled level thanks to birdies at 16 and 17.
The Swede, who has two wins on the Asian Tour, missed the fairway at both play-off holes but scrambled a par at the first and then won the tournament with a cool-headed putt on the second time round at the 18th hole.
It would have been the 12th EPGA victory for the 30-year-old Kaymer from Dusseldorf, who won his first major at the 2010 PGA Championship.
Frenchman Romain Wattel, who shared the overnight lead, bogeyed the 17th and finished alone in 10th and two shots back, while amateur Lorenzo Scalise was the highest placed Italian after a brilliant final round six-under par 66 left him in a tie for 11th.
Leading scores after the fourth and final round of the Italian Open on Sunday at the par-72 Golf Club Milano (GBR % IRL unless stated, x denotes amateur):
269 - Rikard Karlberg (SWE) 67 67 68 67 (Won at the second play-off hole), Martin Kaymer (GER) 68 66 65 70
270 - Lucas Bjerregaard (DEN) 66 65 70 69, Jens Fahrbring (SWE) 67 64 68 71, Fabrizio Zanotti (PAR) 66 67 68 69, David Lipsky (USA) 67 67 68 68, Joakim Lagergren (SWE) 72 66 69 63, Danny Willett 68 67 69 66, Matthew Fitzpatrick 70 68 67 65
271 - Romain Wattel (FRA) 67 66 66 72
272 - Roope Kakko (FIN) 67 72 67 66, (x) Lorenzo Scalise (ITA) 71 66 69 66, Miguel Angel Jimenez (ESP) 69 66 72 65 273 Jason Scrivener (AUS) 68 68 67 70, Merrick Bremner (RSA) 71 68 70 64, Jason Barnes 69 68 69 67, Jaco Van Zyl (RSA) 71 65 66 71, Bernd Wiesberger (AUT) 66 68 67 72, Peter Lawrie 68 67 70 68
274 - Francesco Molinari (ITA) 65 72 68 69, Nicolas Colsaerts (BEL) 63 70 70 71, Ricardo Santos (POR) 69 68 68 69, Tom Lewis 72 67 67 68, Darren Fichardt (RSA) 69 70 69 66
275 - Mikael Lundberg (SWE) 68 68 67 72, Kristoffer Broberg (SWE) 65 72 67 71, David Howell 70 69 70 66, Alejandro Canizares (ESP) 68 69 69 69, Mikko Korhonen (FIN) 66 70 70 69, Benjamin Hebert (FRA) 69 69 69 68, Y.E. Yang (KOR) 68 71 69 67, Florian Fritsch (GER) 68 72 70 65
276 - Jeev Milkha Singh (IND) 68 69 68 71, Shiv Kapur (IND) 65 72 70 69, Daniel Brooks 71 65 67 73, Paul Maddy 68 69 70 69, Tommy Fleetwood 70 67 71 68, Wade Ormsby (AUS) 69 67 74 66, Padraig Harrington 70 70 69 67, Andrea Perrino (ITA) 67 67 74 68
277 - Ricardo Gonzalez (ARG) 68 72 67 70, Stewart Cink (USA) 69 68 70 70, Pedro Oriol (ESP) 68 65 74 70