Eight years after his only previous European Tour victory, the 31-year-old won by a stroke from Italy's Francesco Molinari at Le Golf National, near Paris.
Siem, whose performance also earns him a place in the Open Championship, took Europe's oldest golf title with a superb closing 67 for an eight-under-par total of 276.
He began the week 120th on the rankings, but has moved into the top 60 in one mighty leap and picked up a cheque for just less than £420,000.
Last year's champion Thomas Levet jumped into the lake to celebrate and broke his leg, but thrilled though he was and excitable though he is –nobody has more fist-pumps on the course – Siem sensibly decided to stay out of the water.
"I'm just going to have some nice champagne," he said. "I've had so many tournaments I kind of destroyed this year and I am so happy I got it done.
"It's all about winning in the end and now I'm in the Open, which is unbelievable."
Molinari, who lost a play-off to Miguel Angel Jimenez on this course two years ago, threatened to pull off an astonishing win after double-bogeying the opening hole.
He was eight behind at that point, but then grabbed nine birdies and set the target with a best-of-the-week 64.
Siem was joint leader at the time and responded by splashing out of sand to seven feet at the long 14th, then hitting a glorious iron to four feet on the dangerous next.
There were still three tough holes to come, but he parred the first two and after driving into a bunker on the last achieved the bogey which he knew was probably going to be good enough.
French hope Raphael Jacquelin could have forced sudden death by holing his second to the 18th, but a par-4 left him third on his own on six under. One stroke further back were English trio Ian Poulter, David Lynn and joint overnight leader David Howell.
While Howell could never make amends for bogeying two of the first three holes, Poulter had a share of the lead for a while, but bogeyed the final two holes for a 69.
Molinari's second place lifts him into an automatic qualifying position for the Ryder Cup with only seven weeks of the race to go.
The 10th and final spot has changed the last three weeks. First Nicolas Colsaerts was replaced by Gonzalo Fernandez-Castano, then it was Rafa Cabrera-Bello and now it is Molinari.
"Just a crazy day really," said Molinari. "Two years ago when I was second I double-bogeyed the first on the last day. All of a sudden everything was going in."
Justin Rose came joint ninth and Graeme McDowell was in a tie for 17th, while Lee Westwood climbed 18 places to 40th with a closing 70.
David Drysdale was the leading Scot on 282, two under par, just outside the top 10 after a closing 70, two shots ahead of Marc Warren (71). Scott Jamieson finished on 286 after shooting a 71.
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