The 27-year-old, who has played for Italy four times, scoring on his debut against Albania in November 2014, notched 17 goals for Anderlecht last season.
A statement on the Vicarage Road club's website confirmed he had joined the Hornets on a five-year contract, following last week's triple swoop for Daryl Janmaat, Roberto Pereyra and Younes Kaboul.
Okaka has experience of English football having spent time on loan with Fulham in 2010.
Source : PA
Source: PA