The former Liverpool and Manchester City winger will return to England to aid the Hornets' promotion bid before formally joining Udinese on a free transfer on July 1
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 Zebrette player on July 1 but the Spaniard will spend the next three months at Watford, who, like Udinese, are owned by the Pozzo family.
The Spaniard began his career in his homeland with Mallorca but subsequently went 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 to complete the deal.