The 22-year old forward comes back to the league where he made a name for himself as a teenager who was widely touted as the 'next big thing' in US soccer.
After four years trying to make it in Europe, Adu is back in North America's domestic league and the Union are very happy to have him.
"I think over the past four years abroad, he learned a lot and grew up a lot," Union defender Danny Califf told MLSsoccer.com.
"I'm really excited to see what he brings every day in training. He's definitely a guy that can do uncommon things. And that's the type of guy we need at the moment."
Phillie midfielder Brian Carroll said it was now good to see a little less attraction and fanfare surrounding Adu who can focus more on his game as he matures.
"The fans are always tremendous, but adding Freddy made it even more exciting," Carroll said. "Once he's more acclimated, he'll fit even better. But we're happy to have him and I think he'll be a valuable addition to us.
"It was a fan frenzy back then. It's still as exciting and fun to be around and fun to play with him. Now with him being a little bit older but still a young guy with that much more experience - I think it's exciting for us and the fans."