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.
Also on Wednesday fellow La Liga side Athletic Bilbao said six players had tested positive as they returned for pre-season training.
Valencia confirmed two cases of Covid-19 this week, while Atletico Madrid reported two positive tests before their Champions League quarter-final.
Barca say he their player has not been in contact with any senior players, who travel to Lisbon on Thursday to prepare for their Champions League quarter-final.