The Italian striker did not enjoy the most successful of stints at Vicarage Road, managing just one appearance in a 3-0 home defeat to npower Championship leaders Southampton at the start of his spell.
Trotta, 19, arrived fresh from having bagged eight goals in eight appearances during a loan stay with League One Wycombe, form which earned him his first appearance for Fulham as a late substitute in their January FA Cup defeat at Everton.
Hornets boss Sean Dyche told Watford's official website: "Marcello came in to what was a very tough ask against Southampton and we ended up shuffling things around after that game.
"We've been unbeaten since then so he's been unlucky in that there hasn't been that real need to alter too much.
"I hope he's had some good development here. I know he's learned a lot about Championship football and how it differs from League One. Marcello has been very professional and a credit to himself. We wish him well for the future as he goes back to Fulham."