The 29-year-old has found first-team opportunities limited this term, with duties between the posts largely shared by David De Gea and Anders Lindegaard.
Kuszczak, who began his professional career in Germany with Hertha Berlin, arrived at Old Trafford in July 2006, initially on loan from West Bromwich Albion before the deal was made permanent the following summer.
Tomasz has made 61 appearances for the Reds but will now have his sights set on helping Watford, currently in 14th place in the Championship, avoid relegation. He will face a battle for the gloves, though. Current Hornets stopper Scott Loach, a former England Under-21 regular who was also named in Fabio Capello's senior squad in August 2010, is almost ever-present between the posts at Vicarage Road, playing 31 of the Hornets' 35 fixtures so far this season.