Antonio's return to English football comes almost two years after his final senior appearance for West Ham, in a 3-1 defeat by Leicester City on 3 December 2024, four days before he broke his leg in a car crash.
The 36-year-old, Watford's 12th signing of the summer, has agreed a one-year deal at Vicarage Road after spending the past month training with Alessio Dionisi's squad in an effort to prove his fitness following an extended lay-off.
"Watford is a big club and hopefully I can come in and bring success and help wherever I can," Antonio told the club's website.
The move is subject to international clearance, which would need to be completed in time for Antonio to feature in Saturday's Championship home fixture against Stoke City.
West Ham United released Antonio in June 2025 at the end of a 10-year spell during which he became the club's all-time leading Premier League goalscorer with 68 goals, having made 323 appearances in all competitions for the Hammers.
He returned to the London Stadium as part of his rehabilitation from the car accident and made two appearances for the under-21 side in Premier League 2, before a brief spell in Qatar with Al-Sailiya earlier this year, where he made four appearances without scoring or assisting.
His route back to English football was not straightforward. Antonio trained with Leicester City in December and had been on the verge of signing for Brentford last season, only for a torn calf muscle sustained the day before his planned move to derail the deal entirely.
Before joining West Ham in September 2015 in a £9.5 million move from Nottingham Forest, Antonio came through the EFL with earlier spells at Reading and Sheffield Wednesday, having started his career at non-league side Tooting & Mitcham United.
He also helped West Ham win the Europa Conference League in 2023, ending a 43-year wait for major silverware at the club, and made three substitute appearances for Jamaica at this summer's Gold Cup as he continued his recovery.
Antonio, who will wear the number 18 shirt, joins a Watford side sitting 12th in the Championship having won, drawn and lost two of their opening six league games under Dionisi, as the club looks to add experienced firepower to its attack ahead of the winter months.