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 Udinese, are owned by the Pozzi family.
The Spaniard began his career in his homeland with Mallorca and has gone on to play for Bordeaux, Espanyol, Manchester City, Liverpool and Olympiacos.
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.
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