The 27-year-old has agreed a three-year deal, with the Toffees having the option of a fourth season.
Doucoure becomes the third midfielder to sign for Carlo Ancleotti’s men in the past week after the arrivals of James Rodriguez from Real Madrid and Brazil’s Allan from Napoli.
“I have been waiting a long time to come here and now it has happened so I am very happy,” said Doucoure.
“I had some interest from other clubs but from the beginning when Everton showed an interest in me I told my agent to focus on Everton. It was the only club I wanted to join.”
The Frenchman played 129 Premier League games in four seasons at Vicarage Road and was directly involved in 29 goals, scoring 17 and assisting 12.
“I can bring my energy – I am a powerful player, I can run a lot, all the game,” said Doucoure.
“I can add goals as well. I will try my best to score goals and help the team.
“It is an ambitious project here and it was a big part of my decision, having (Director of Football) Marcel (Brands) and the coach Carlo Ancelotti wanting me.
“When you come to a club you want the manager and, of course, the Board to support you.”
Everton begin their Premier League campaign at Tottenham on Sunday.
