The Union reaquired forward Sebastien Le Toux on Thursday by sending allocation money and Costa Rican forward Josue Martinez to the New York Red Bulls.
Specific terms of the deal were not announced.
"We are happy to welcome Sebastien back to the club," manager John Hackworth said. "His determination and heart are a perfect fit for the mentality of this team."
Le Toux, 28, was originally acquired by the Union as the fourth overall pick from Seattle Sounders FC in the 2009 MLS expansion draft. In his two seasons with the Union, the Frenchman led the team in goals and was instrumental in the club's 2011 MLS playoff run.
In 62 total appearances (28 in 2010, 34 in 2011), he is the Union's all-time leading goal-scorer with 25 goals and 20 assists. He is the only Union player to net a playoff goal, scoring in the first leg of the 2011 MLS Eastern Conference semifinals against Houston.
Le Toux also collected MLS's individual fair play award in back-to-back seasons, going three consecutive years without a carded infraction.
"In our hearts Sebastien has always been a Union player, and his contributions on the field and in the community while he was in Philadelphia were invaluable. We are thrilled to welcome him back and look forward to having him rejoin the club," CEO and operating partner Nick Sakiewicz said.