The 32-year-old midfielder had trained alone or with the club's under-23s since Jose Mourinho was appointed United boss in the summer.
Former Germany captain Schweinsteiger last played for the Red Devils in March, when Louis van Gaal was manager.
He said in August he had "no personal problems" with Mourinho, and United will be his last club in Europe.
"It's great to see him back," said United left-back Luke Shaw. "We found out the news a couple of days ago.
"He's a big influence in the dressing room and obviously on the pitch, especially for the young players like myself."
It is unclear whether Mourinho will now consider Schweinsteiger for first-team action.
He was named in United's Premier League squad for this season, but left out of their Europa League squad.
Schweinsteiger has made 31 appearances for the club since his £14.4m move from Bayern Munich in July 2015.
Simon Stone, BBC Sport
Before Schweinsteiger can think about a return he still hoped for during the dark days of being forced to train on his own, he needs to know why Mourinho brought him back into the fold.
After all, he cannot play against Fenerbahce on Thursday because he is not in Mourinho's Europa League squad.
With Ander Herrera banned, Schweinsteiger is an option for United's Premier League trip to Swansea.
However, given he is so short of match practice, is that really an option? Or is Mourinho merely reminding potential January suitors that the 32-year-old still exists?
World Cup winner Bastian Schweinsteiger has returned to training with Manchester United's first team.