The World No 1 became Italy's first male winner of the tournament since Adriano Panatta in 1976.
Sinner's triumph on the red clay of the Foro Italico also made him only the second man after Novak Djokovic to win all nine Masters 1000 events - the biggest tournaments outside the Grand Slams - and complete a 'Golden Masters'.
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Djokovic completed the career set in 2018 in Cincinnati at age 31 - and then went on to win each event at least twice.
Sinner is 24, and with his only real rival, Carlos Alcaraz, currently sidelined due to a right wrist injury, seemingly nobody can beat him.
Sinner also extended his winning streak to 29 matches, with his most recent defeat having come against Jakub Mensik in the Qatar Open quarterfinals on February 19.
Having also claimed Masters 1000 titles in Monte-Carlo and Madrid, Sinner is now 17-0 on clay this year entering the French Open, which starts next Sunday.
"Incredible," Sinner said. "It's been a long time since an Italian won - 50 years. I'm happy one of us was able to take advantage of this great period for Italian tennis."
Sinner celebrated calmly as usual, revealing a wide smile when he landed an inside-out forehand on the line on his first championship point.
He then waved to the crowd, which included Panatta sitting in the front row.
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