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The actor, 65, was at the glitzy festival in France for the screening of his new film, Elvis.
He looked slender in his smart black suit and bow tie as he posed for pictures alongside cast members including Austin Butler, director Baz Luhrmann and Elvis Presley's former wife Priscilla.
And fans were blown away by how much weight he seemed to have lost.
"Tom Hanks looking very skinny on this Elvis red carpet," one person posted on Twitter.
"Wow!" said another, adding that the Hollywood star looked "so thin".
"Damn Tom Hanks lost so much weight," one fan tweeted.
"Tom Hanks looks so skinny," said another.
One person joked that the man on the red carpet must actually have been someone else.
"This is absolutely NOT TOM HANKS!!!" they posted, quipping that it could be the star's twin.
The actor has previously changed his weight for film roles.
He reportedly lost 50lbs to play a man stranded on a desert island in the 2000 movie Castaway, and gained some weight for 1992's A League Of Their Own.
However, the star has type 2 diabetes and has previously that said he would no longer take on roles that required him to change his weight.
"I think that's more or less a young man's game," he said in 2013.
Elvis explores the life and music of the late king of rock 'n' roll, who is played by Butler.
Hanks stars as his manager, Colonel Tom Parker, and Olivia DeJonge plays his wife Priscilla.
Watch: Cannes Film Festival all shook up by Elvis premiere