The 31-year-old has left the Serie A runners-up to try his luck in the Premier League and will stay at Vicarage Road on a deal until 2018.
Watford confirmed the news on Thursday night in a tweet which read: “Greece international defender Jose Holebas signs a 3-yr deal with #watfordfc after joining from Italy Serie A runners-up AS Roma.”
The arrival of Germany-born Holebas came on the day former Manchester United defender Craig Cathcart signed a new contract at Vicarage Road.
The 26-year-old Northern Ireland international, who made 30 appearances for the Hornets last season as they won promotion to the Premier League, signed a new four-year deal.
Watford, under new head coach Quique Sanchez Flores, have been busy during the close-season after Matej Vydra has turned his loan move from Italian side Udinese into a permanent deal and Gabriele Angella has penned a new five-year deal.