Udine announced the signing of the Spaniard in March on a pre-contract agreement after he was released by Turkish giants Galatasaray.
Riera spent the remainder of last season on loan at Watford and linked up with Andrea Stramaccioni's men when his contract started on July 1.
The much-travelled 32-year-old has now become a free agent after failing to make an impact at Stadio Communale Friuli.
Espanyol, Manchester City, Liverpool and Olympiacos are among the clubs Riera has turned out for over the years.
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