Jurado, 29, and Britos, 30, have both agreed three-year contracts with the Hornets, who won promotion back to the Barclays Premier League last season.
Jurado played under Watford head coach Quique Flores during four years at Atletico Madrid, helping the Spanish side win the Europa League in 2010, before enjoying spells at Schalke in Germany and Spartak.
Britos moved to Napoli from Bologna and helped the Italian outfit finish second in Serie A in 2013, and also won the Coppa Italia twice.
The Uruguayan, however, was sent off in what turned out to be his final appearance for the Naples club after headbutting Alvaro Morata during the closing minutes of the 3-1 defeat at Juventus on May 23.
The arrivals of Jurado and Britos mean Watford have now signed nine players this summer after Sebastian Prodl, Giedrius Arlauskis, Matej Vydra, Jose Holebas, Etienne Capoue, Valon Behrami and Allan Nyom penned permanent deals at Vicarage Road.