The 36-year-old brings extensive Premier League and Championship experience, as well as European football, to Vicarage Road.
Head coach Alessio Dionisi believes Antonio’s experience will complement the Championship’s youngest average starting XI.
The forward is expected to contribute on the pitch while also offering valuable mentoring and coaching support to Watford’s young squad.
“It’s a good set of boys here,” Antonio told the club website.
“They are focused and ready to show their abilities this season.
“Watford is a big club and hopefully I can come in and bring success and help wherever I can.
“I’ve scored a few goals against Watford! So hopefully I can score some for Watford now when I’m fully up to speed."