Brown made his Quins debut in 2005 and has gone on to play 348 times for the club, scoring 98 tries.
The 35-year-old was part of the side that won the Premiership title in 2011/12 and represented England 72 times.
“I want to take this opportunity to thank the people that have made my time during Quins a special part of my life,” Brown said in a statement released by the club. “Firstly, to the fans that have supported me so fiercely. I, and the team, have missed you hugely over the last few months and I hope I get the opportunity to play in front of you at The Stoop once again.
“I also want to thank the special people who gave me my chance as a young player. Hopefully, I’ve repaid that faith during my time in the Quarters to date and for the rest of this season.
“I’ve been fortunate enough to play alongside some great players, in some great teams, but more importantly I have met many great people during my time with Harlequins.
“While this will be a big change for me, I remain fully focused on continuing to give my all on the pitch alongside some great friends this season. This is a strong group and I fully believe we can achieve something great this year.”