The 28-year-old from Paisley won the Fiat and Fila Italian Open here by one stroke from his former Walker Cup team-mate Padraig Harrington.
Robertson claimed the pounds 119,042 first prize with a closing round of 68 for a 17-under-par total of 271. Remarkably, he is the seventh first-time winner in the last 10 Tour events and, even more amazingly, he had missed the halfway cut in eight of his 11 starts this season.
The American Open champion, Lee Janzen, on a rare visit to Europe, finished only 66th and Robertson said: "I think Mr Janzen would have got a surprise about the depth of the European Tour this week. My legs were shaking the last few holes, but I held my nerve. I told some friends a few weeks ago that despite my record this year I'd never hit the ball so well. I said that at the odds (over 100-1), I was worth a bet. I hope they did it!"
Among the first people on the telephone was his manager, Ian Doyle, from the Crucible in Sheffield. "It's four years since I signed up Dean and I always knew it was only a question of time," Doyle said. "I'm delighted for him - he's worked so hard."
Dubliner Harrington, who had led by one stroke overnight, recorded bogeys at the 16th and 17th holes, but Robertson missed the chance to go three clear by taking three putts at the 17th.
Harrington then made a 15-foot birdie putt on the last, but by then the Canadian-born Scot had putted to less than 18 inches from the flag, and he knocked in the title-winning putt.
Harrington's putt broke a four-way tie for second place with the English pair Gary Evans and Russell Claydon and also the Welshman Phil Price, who had finished with a superb 63. The putt was worth nearly pounds 32,000 - he took home pounds 79,357 rather than pounds 47,517 - but he said: "It didn't count for much. I wanted to win. But Dean played well and thoroughly deserved to win."
Meanwhile, the Masters champion, Jose Maria Olazabal, closed with a 66 for joint 13th spot and Seve Ballesteros had his best round for almost a year and his best finish for even longer. Having failed to make a single cut all season until last week's Spanish Open, Ballesteros finished 17th after a closing 67.
The 42-year-old, who has been fighting a stiff neck this week as well as knee and back problems, still entertained thoughts of winning when he turned around in 31.
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