This is the first time Lionel Messi will be training with Barcelona after his epic summer transfer saga.
The Argentine forward, 33, was seen arriving at Ciutat Esportiva on Monday after announcing he is reluctantly staying put despite handing in a transfer request last month.
Messi arrived 90 minutes early for the session with new boss Ronald Koeman.
He underwent medical tests on Sunday to make sure he was OK to come back.
And Messi was the first Barcelona player to arrive for Monday’s behind-closed-doors session.
The forward’s return marks a sensational U-turn from his position just one week ago, when he missed all of Koeman’s first training sessions.
Messi demanded to leave Barcelona in the wake of the 8-2 Champions League exit to Bayern Munich, blasting the club’s hierarchy and pointing to mismanagement of teammates including close pal Luis Suarez.
He believed he still had a clause valid in his contract until the end of the season allowing him to depart Catalonia on a free transfer.
But Barcelona argued that the clause expired in July and was quickly given backing by LaLiga.
It led to Messi confirming last week that he will be staying at the club for the final year of his contract, as the only way to press on with the potential free transfer was by taking Barcelona to court.
Paris Saint-Germain has since confirmed they were also plotting a stunning move to reunite Messi with old pal Neymar.
But with the veteran now back in training, Barcelona will be looking to move on from the ordeal and challenge rivals Real Madrid for their title.
Speaking about Messi’s decision to stay, LaLiga president Javier Tebas beamed: “We were worried but not seriously.
“We want Messi to be with us, he is the best player in the history of football and we want him to end his career in LaLiga.
“We’re pleased he is staying with us and not going to another league.”