Barcelona say an unnamed squad member has tested positive for Covid-19.
He was one of nine reporting for pre-season training, showed no symptoms and has been quarantined at his home.
Barca say he has not been in contact with any senior players, who travel to Lisbon on Thursday to prepare for a Champions League quarter-final.
Valencia confirmed two cases of Covid-19 this week, while Atletico Madrid reported two positive tests before their Champions League quarter-final.
Because of the coronavirus pandemic, the Champions League is concluding with a 'final eight' tournament in Lisbon, comprising single-leg games held behind closed doors.
Barcelona play Bayern Munich on Friday, the day after Atletico Madrid face RB Leipzig.
Everyone who has been in contact with the Barcelona player has been tracked to perform tests.