Messi was seen driving into the Ciutat Esportiva in Sant Joan Despi at just after 4:00 pm (1400 GMT) on Monday, where he is expected to take part in training for the first time under new coach Ronald Koeman.
If he has passed his coronavirus test, Messi could also begin by training alone before joining the rest of the group later in the week.
The Argentinian superstar said on Friday in his explosive interview with Goal that he would continue to give his best for Barca, despite admitting defeat in his bid to depart for free.
The 33-year-old maintained he tried to go because he "wanted to live his last years of football happily" while also blaming the club's president Josep Maria Bartomeu for "breaking his word" and forcing him to stay.
The return of Barcelona's embittered captain and greatest ever player may not be an easy one, particularly given some of his closest team-mates are likely to be absent.
Luis Suarez, Messi's best friend and neighbour, as well as Arturo Vidal, are expected to join Juventus and Inter Milan respectively.
It will also be the first time Messi has trained under Koeman, who was appointed Barca's new coach last month.
"Now I don't know what will happen," Messi said on Friday.
"There is a new coach and new ideas. That's good, but then we have to see how the team responds and whether or not it will allow us to compete at the top level. What I can say is that I'm staying and I'm going to give my best for Barcelona."
Long-term future in doubt
Messi's decision to notify the club via burofax that he intended to leave came after he met with Koeman, talks that reportedly involved Dutchman taking a hard line with his top striker.
Koeman said in his first press conference that he "only wanted players who want to be here and to give everything".
And Messi refused to attend either a coronavirus test or Koeman's first week of pre-season training last week, as Messi's lawyers believed his participation would harm his case in a potential legal battle with the club.
But Messi opted not to go down that route, insisting he could never go to court against "the club of my life".
Yet his long-term future remains in doubt given Messi can talk to other clubs in January and leave for free when his contract expires on June 30.
If Messi passes his coronavirus test and restarts training without problems, he could feature in Barcelona's friendly against Gimnastic Tarragona on Saturday.
A more realistic reintroduction might be against Girona four days later.
It would be the first time Messi has played for the team since their historic 8-2 defeat by Bayern Munich in last month's Champions League quarter-finals.
Barcelona have been given an extra two weeks off to recover before they start their La Liga season against Villarreal, scheduled for September 27.