Briggs has found it hard to break into the first-team since making his senior debut as a 16-year-old in May 2007 as a substitute against Middlesbrough.
He has again found it difficult to dislodge John Arne Riise this season from the starting left-back slot and made four appearances for Bristol City in October on loan before returning to Craven Cottage.
The left-back, who has made eight appearances for the Whites this season, the last in the recent defeat by Manchester City will remain with the npower Championship side until the end of the season.
Fulham manager Martin Jol said: "If you look at the competition for places in my team at left-back, you've got John Arne Riise, Kieran Richardson and Urby Emanuelson who can play there.
"Briggs is there too and he's making good steps, but I feel that in the next four weeks he will only play two games probably and for him it's important that he gets games under his belt.
"So that is the idea behind sending Briggs on loan to Watford."