Molmenti finished 10th in this event at the 2008 Olympics, but in the four years since has won a world championship and three European titles.
He fought through the churning water of the slalom course in 1:33.43 to defeat Vavrinec Hradilek of the Czech Republic by 1.35 seconds.
Hannes Aigner of Germany finished just 0.14 second behind Hradilek to win the bronze.
The medal won by Hradilek was the first for the Czech Republic at the London Olympics.
Costly malfunction causes beer flood at Boston-area brewery
Video of Victoria’s Secret models trying to 'twerk' hits Instagram