The 31-year-old is currently on international duty at the World Cup, and started Samoa's Pool D games against Namibia and Wales.
He joined Quins from Super League side St Helens at the start of last season, and made a big impression as the London club went on to win the Amlin Challenge Cup.
And director of rugby Conor O'Shea is keen to see Fa'asavalu back in a Quins shirt once his commitments in New Zealand are over.
He said: "Maurie has made a huge impact on both players and supporters alike and his example as a professional in everything he does is one which all young players should follow.
"He is a delight to have around. There is no doubt he brings something different and will help us develop further as a group. We look forward to having him back after the Rugby World Cup."
Fa'asavalu added: "I am really excited to have re-signed with Harlequins, from the end of this season, for another two years.
"It's great to be part of a squad that is continually growing and where everyone is working hard to achieve our goals to bring silverware to the club.
"My family has really settled in down here and we enjoy the lifestyle, and the welcoming and inclusive nature of Harlequins makes it a great all round."
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