The 33-year-old former Sunderland centre-back has played just six matches for his parent club this campaign and heads to Ashton Gate in search of regular football.
The Robins have the option to extend Nosworthy's stay until the end of the season.
Manager Steve Cotterill told the club's official website: "He's someone I've had in the back of my mind since doing well against us in the FA Cup and we've built up a good relationship with Watford.
"He's a good lad and has fitted in well already with the lads in training today. It's great we've been able to get him on board."