The England back-rower, who has been in fine form this season, joined the Aviva Premiership leaders in 2004 and has scored 42 tries in 162 appearances.
"Harlequins is a club that allows you to express yourself on the pitch," the 33-year-old said.
"We're playing an attractive style of rugby that is not only producing positive results, it's also enjoyable to be part of.
"I'm looking forward to contributing to the ongoing success of the team and to winning more silverware for the club and its supporters."
Easter has been capped 47 times but was omitted from England's elite player squad for the Six Nations.
However, Quins director of rugby Conor O'Shea believes he is the country's most accomplished performer in his position and still has plenty to offer.
"I believe he is currently the best number eight in the country," said O'Shea.
"Nick still has a lot that he wants to achieve in the game.
"Having come to professional rugby later than others, and given the condition he keeps himself in, we believe he will play at the highest level for years to come."
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The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
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The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.