According to Access Hollywood, Fallon, 32, proposed to his girlfriend, Nancy Juvonen, 40.
The actor proposed while the couple was at the Juvonen family cottage in New Hampshire, last weekend.
The couple worked together in 2005 in the romantic comedy Fever Pitch, which was produced by Flower Films. Juvonen is part-owner of Flower Films, also co-owned by Drew Barrymore.
It is the first marriage for both Fallon and Juvonen.