Sissoko has signed a two-year contract at Vicarage Road and could ironically make his debut against Tottenham in the Premier League on Sunday.
The 32-year-old France international was Tottenham’s club record signing when he joined from Newcastle United for £30m in the summer of 2016.
Sissoko made 202 appearances across all competitions for Spurs and helped the North London outfit reach the UEFA Champions League final in 2019, losing to Liverpool.
Watford announced: "The Hornets are delighted to announce the signing of vastly experienced France international Moussa Sissoko from Tottenham Hotspur.
"Sissoko, who has 70 international caps and 259 Premier League appearances to his name, has agreed a two-year contract with Watford and is available for Sunday’s meeting with former club Spurs.
"The midfielder broke through at Toulouse and earned a move to England with Newcastle United in 2013, going on to make 133 appearances for the Magpies ahead of a £30m transfer to Tottenham."
Speaking ahead of Sissoko’s arrival on Friday, Watford manager Xisco Munoz said: "He is a very good player, everyone knows he is an experienced player.
"He is a top player and I think he would be very good for us. We will see if he arrives. Always I say the same thing - we will have one option more in our squad to try to give all the best for our team."
Sissoko was part of the France squad at EURO 2020.
