The French star took to Facebook.com to share his grief and offer a tribute to Gandolfini, who is believed to have suffered a fatal heart attack in Rome on Wednesday (19Jun13).
Marini last saw his friend just one week ago at a school event for their children, and reveals the 51 year old was "so happy" about bringing his young son Michael, from his first marriage to Marcy Wudarski, with him to Italy for a special "boy trip".
He writes, "I am heartbroken. My bud James Gandolfini just died. Last Saturday he told me at our kids' graduation ghat (that) he was so happy to go with his son to Italy. A boy trip!
"It was an honour to have met this man, such a great dad! I spent so much time with James' son teaching him soccer. I feel for that kid... it must be so hard right now for little Micheal (sic). Guys say a prayer for him and his family. This is going to be (a) very difficult time for them. I am so so sad! RIP brother... you'll be missed!"
The actor was in Italy with his son and sister, and reports suggest they had just returned to their hotel after a day of sightseeing when Gandolfini collapsed in the bathroom.
The New York Post reports Gandolfini's sister found him unconscious on the bathroom floor and called an ambulance, but paramedics were unable to save him. He was pronounced dead on arrival at a local hospital.
Gandolfini, who was scheduled to appear at Italy's Taormina Film Fest this weekend (22-23Jun13), also has a baby daughter, Liliana, with his second wife Deborah Lin, who gave birth to the tot last October (12).
Since news of his death broke, fans have been leaving floral tributes outside Gandolfini's former home in New York City.