The 24-year-old England World Cup hopeful's new deal will keep him at the Twickenham Stoop until at least 2014.
Robshaw, capped by England in 2009, skippered his country in tour games against the Australian Barbarians and New Zealand Maori last summer.
He is a Quins academy product, and in his first season as club captain Quins have reached the Amlin Challenge Cup final, when they will play Stade Francais in Cardiff on Friday week.
Robshaw, a former Premiership player of the season, said: "Harlequins are a great club, and one which I think is on the verge of great things in the coming years.
"We have a squad with a great mix of youth and experience, and we are trying to play a brand of rugby that will excite and challenge."
Quins rugby director Conor O'Shea added: "Chris Robshaw embodies everything this club is and wants to be about.
"At still only 24, Chris wants to push on for both Harlequins and England, two goals that are well within his compass."
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