The 36-year-old joined Quins in 2008 and has since gone on to make more than 200 appearances, scoring a club-record 2,217 points.
Evans also won 16 caps for New Zealand from 2004 to 2007 and is widely regarded as one of the greatest overseas signings in English rugby history.
"I thought leaving New Zealand was hard, but leaving the game I've played since I was eight years old, and professionally for 17 years, is tough. But when you know, you know. This game has taught me everything, showed me things I could never imagine and introduced me to people that are not teammates but family," he said in a statement on the club's official website.
"I need to thank the clubs I have represented, the coaches who have helped shape the player I am today and the fans, both in New Zealand where it all started, and here in the UK, for their support and cheers. I will never be able to express how much representing the All Blacks, and living my childhood dream, meant to me.
"I want to thank my parents and brother who are my biggest fans, and thank them for all the support, trips to training and games, and their "constructive criticism" as only family can give. But most importantly my wife Sally and kids Olivia, Harry and Billy, thank you for being there through good and bad and making it easy to play the game I love; your support has been amazing.
"I couldn't think of a better shirt and place to finish my career at than Harlequins, I want to thank the club and fans for making myself and my family feel unbelievably welcome; it has made our time so special. Importantly I have to also thank the players for keeping me going over my time here. I am only a little cog in the Harlequins machine and I couldn't have done anything without my mates beside me on the field."
Quins director of rugby John Kingston added: "The word 'legendary' is really over used within the modern sporting world, but in the case of Nick Evans and his rugby playing career at Harlequins, it is the perfect description.
"From the moment he arrived in the summer of 2008, he has been at the forefront of taking the club to heights never experienced before, including being crowned Aviva Premiership champions in 2012. Not only has his on-field play been outstanding, but he has set the ultimate standards of professionalism in everything asked of him during his time here.
"It is a huge statement to make but I genuinely question if any player in the history of the club has had such an impact on team performances. I have no doubt he will make his decision on 'what next?' in his usual diligent manner but whatever it is, I wish him, Sally and his children all the very best while thanking him immensely for his incredible contribution to Harlequins."
Evans's final game in a Harlequins shirt is likely to come away to Northampton Saints on May 6.