Messi had failed to turn up to the club's tests for COVID-19 eight days ago and been absent from all subsequent sessions ahead of the new campaign after giving the club an official notice that he wished to leave.
But the club's all-time top scorer finally appeared at the Joan Gamper training ground ahead of an evening session, his first under new coach Ronald Koeman, after taking a test for the novel coronavirus at his home on Sunday.
Messi, 33, revealed last Friday (September 4) in an interview with Goal.com that he was staying at Barca for the coming season as he did not wish to face a court case with the club where he has spent his entire career over a disputed release clause in his contract.
The 33-year-old Argentine forward took a swipe at the club's leadership during the announcement that he would stay.
"The management of the club led by (president Josep Maria) Bartomeu is a disaster," he was quoted as saying on Goal.com.