"Whether you are a coach, player or supporter of Harlequins, this is fantastic news for everyone at the club," said Harlequins director of rugby Conor O'Shea.
"Nick is a world-class player who fits everything that this club wants to be and is also hugely motivated to win silverware with Quins. I am also sure he will continue to help in the development of the 20-year-old Rory Clegg."
Evans clocked up 11 European appearances in his first two seasons with Quins, scoring 75 points in the process, none more valuable than the memorable last-minute drop goal against Stade Francois in 2008 that helped Harlequins into the quarter finals of the Heineken Cup for the first time in over a decade.
"Harlequins allow me to play the style of rugby that I believe brings out the best in me as a player," said Evans.
"We have a great team here at Quins, right the way down from the international and senior players, to the academy boys who are breaking through, to the coaches."
"I am happy both on and off the pitch and am looking forward over the next two years to helping the club compete for those top-table places and challenge for silverware."
Harlequins begin the Amlin Challenge Cup away to Bayonne on Saturday, October 10, and then host Italian side I Cavalieri Estra before back-to-back games against Irish province Connacht in December.