The 31-year-old, who played in the Premier League for Liverpool and Manchester City, will return to England to aid the Hornets' fringe promotion bid
The 31-year-old signed for Udinese as a free agent on Monday, having cancelled his Galatasaray contract by mutual consent in January.
He will officially become a Udinese player on July 1 but the Spaniard has now moved to Watford in the short-term, who, like the Italian club, are owned by the Pozzo family.
The Spaniard began his career in his homeland with Mallorca and has gone on to play for Bordeaux, Espanyol, Manchester City, Liverpool and Olympiakos.
Riera moved to Galatasaray in 2011 but made just four league starts for the Turkish champions this season.
His transfer to Watford remains subject to the club receiving international clearance.