Bergoglio, who took the name Pope Francis when he was elected to the post on Wednesday, received his San Lorenzo membership card after saying mass at a sports complex near the team's stadium.
"He says he lives in a permanent state of suffering for San Lorenzo," said Oscar Lucchini, an architect and fellow fan who handed Bergoglio's membership card to him after that mass about four years ago.
The club posted a picture on its website on Wednesday that showed a smiling Bergoglio lifting up the club's colors.
As archbishop of Buenos Aires, Bergoglio was known for his unassuming lifestyle. He lives near San Lorenzo's headquarters and has long traveled the capital city by public bus.
He also shares the pain of his fellow San Lorenzo fans after their defeats, as well as their joy when the team wins. The club last won Argentina's national championship in 2007.
(Reporting by Alejandro Lifschitz; Writing by Hugh Bronstein; Editing by Kieran Murray and)