James Gandolfini -- who famously played Tony Soprano on "The Sopranos" -- died earlier today in Italy ... TMZ has learned.

Gandolfini is believed to have suffered a heart attack. He was 51.

Gandolfini was in Italy to attend the 59th Taormina Film Festival in Sicily -- and he was scheduled to participate in a festival event this weekend with Italian director Gabriele Muccino.

Gandolfini shot to fame playing a hitman in the 1993 hit "True Romance" ... and quickly became a Hollywood legend when he was cast as Tony Soprano in 1999. He won 3 Emmy awards for the role during the show's 6 season run.