Freire, the winner here in 2004 and 2007, surged ahead in the final stretch to outsprint Belgian Tom Boonen.
Spain's Oscar Freire surprised the favourites to win the Milan-San Remo classic for the third time on Saturday.
Freire, the winner here in 2004 and 2007, surged ahead in the final stretch to outsprint Belgian Tom Boonen.
Italian Alessandro Petacchi finished third after a 298km effort in the first big classic race of the season.
The 34-year-old Freire, world champion in 1999, 2001 and 2004, was one of several sprinters well placed in the peloton after the Cipressa and Poggio climbs.
The Rabobank rider burst ahead of Italian Daniele Bennati, with Boonen, rated as the top favourite, unable to sustain the pace.
Last year's winner Mark Cavendish, who has been looking out of form this season, was dropped on the Cipressa climb some 25km from the finish line.
For long periods of the race, Freire hid in the warmth of the peloton until the final kilometres as Italian champion Filippo Pozzato tried his luck solo.
"I feared I would not be able to rejoin Filippo Pozzato in the final kilometre," Freire told a news conference.
"I couldn't catch him by myself, other teams had to close the gap for me," added Freire, after the Italian was eventually caught by the favourites' group.
"In the final sprint I was afraid to be boxed in like two years ago but I found the right wheel. I followed Daniele Bennati who was not in great shape."
The promising Edvald Boasson Hagen of Norway, who won the final stage of the Tirreno-Adriatico race earlier this month, suffered a stomach upset and gave up his winning bid for his new Sky Team in the Cipressa climb.
"He's a very strong rider but most of the experienced sprinters did not take part in the sprint of the last Tirreno-Adriatico stage," said Freire.
"The important race was today's. There is a new generation of cyclists coming of age but old riders like me are still competitive. I've won four out of the five sprints I''ve contested this year."
In 2009, Freire announced his possible retirement but he changed his mind and promised that he would call it quits at the end of next year.
"My 14th season will for sure be the last one," he said.
Seven-times Tour de France champion Lance Armstrong missed the race because of gastroenteritis and opted for a training session in southern France ahead of next weekend's Criterium International, where he will face rival Alberto Contador.
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