The 36-year-old midfielder will step away from soccer following the Union's regular season finale against Orlando City on Sunday at Talen Energy Stadium.
"Brian Carroll is a consummate professional, meticulous in his preparation, and a true icon in both Philadelphia Union and MLS history," said Union sporting director Earnie Stewart said in a news release. "His veteran leadership has meant so much for our young players both on and off the field, and the trophies that he has won in this league speak for themselves in terms of his quality."
Carroll is tied for fourth in MLS history with 345 appearances, trailing only Kyle Beckerman, Brad Davis and Steve Ralston, though he has not seen the field for Philadelphia during the 2017 MLS campaign.
The Virginia native has won a pair of MLS Cup titles, with United in 2004 and the Crew in 2008. Carroll also has the distinction of being the only player in MLS history to claim four consecutive Supporters' Shields, winning the title in 2005 and 2006 with D.C. and in 2007 and 2008 with Columbus.
"As I look back on 15 years in MLS, it’s been a fun ride: I've had a lot of great teammates and great coaches, and I had the privilege to represent a number of great cities and clubs," Carroll said. "I'm extremely thankful for the opportunity, and I really enjoyed it. It was fun to win two MLS Cups and four Supporters' Shields, and along the way, to do my best to help grow the game and Major League Soccer."
Carroll, who earned eight caps with the U.S. national team from 2005 to 2010, said Thursday that he plans to move to Indianapolis to pursue a career in financial planning.