The French midfielder has joined the Toffees in a deal believed to be worth around £22million after four seasons with the Hornets. He has signed a three-year deal, with an option for a fourth.
He will wear the No 16 shirt for Everton and revealed to the club website that he had been 'waiting a long time' to join the Merseyside outfit.
Everton have confirmed the signing of Abdoulaye Doucoure from relegated Watford for £22m
The French midfielder has joined the Toffees on a three-year deal with an option for a fourth
'I have been waiting a long time to come here and now it has happened so I am very happy,' Doucoure told evertonfc.com.
'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. Everton has a big history in English football and it is a good club for me to have a step up in my career.
He will wear the No 16 shirt for Everton and revealed he had been 'waiting a long time' to join
'I am very happy now to be an Everton player and very excited for the future.'
Doucoure had been linked with a move to French side Paris Saint-Germain back in January, but remained at Watford as he played a key part in Nigel Pearson's set-up as they fought away in the Premier League relegation battle.
Doucoure topped off busy week for Carlo Ancelotti after signing James Rodriguez and Allan
The all-rounded midfielder will also join up with new signing Allan, who manager Carlo Ancelotti signed from Napoli last week for £21million.
The Italian manager has been bullish in his approach to the new season, and believes the signing of Allan is a step in the right direction.
'We want to make progress,' Ancelotti said. 'The position last season was not enough.
Doucoure was a key part of the Watford set-up in the Premier League but they were relegated
'We want to improve and we are trying to do everything to improve the squad to be better next season.
'I am sure the fans will love Allan's characteristics. And I am sure the fans are also going to love our season.
'Allan is very aggressive. Defensively, he is really good. My personal opinion is that he is one of the best defensive midfielders in the world.
'He is intelligent tactically and he has got a lot of energy. He is a player whose energy can make the others better.'