Former Gunner Almunia signed for the Hornets in 2012, captaining the side to the play-off final in 2013, before a mid-table finish in 2014 was the deciding factor for the 37-year old goalkeeper to attempt to finish his career in his native Spain.
The Spaniard featured 109 times for Arsenal over an eight year period, winning the 2005 FA Cup as an unused substitute, as well as appearing as a 20th minute substitute in the 2006 Champions League Final, following the dismissal of Jens Lehmann.
Since arriving at Vicarage Road, Almunia appeared 76 times, and, despite gaining a reputation as an error-prone goalkeeper, he was a reliable professional, and very quickly gained the backing of the Hornets' faithful.
Albert Riera, who had two separate spells in the Premier League, with Manchester City and Liverpool, joined Watford in March 2014 - despite signing a pre-contract with Udinese Calcio.
Although he agreed to join the Italians, the co-ownership deal between the two clubs meant that the 32-year old winger would join the Hornets until the end of the season. Despite playing only eight games, and scoring one goal, for the Hornets, Riera impressed with his technical ability, and he is certain to have a successful season with the Bianconeri next year.
The winger appeared 65 times in the Premier League, with his longest spell coming with Liverpool, before moving to Olympiacos and then Galatasaray.
On the official club statement, the other five released players include academy graduate Ross Jenkins, ageing centre-backs Nyron Nosworthy, Fitz Hall and Lucas Neill. Former Roma and Italy defender Marco Cassetti is also leaving the club to return to Italy.
Goalkeeper Gary Woods, who signed in 2013 from Doncaster Rovers, is set to leave this summer, although club officials are still in talks to extend his contract.