The 27-year-old midfielder joins James Rodriguez and Allan as new additions to Carlo Ancelotti's squad this week.
After spending four years at Vicarage Road, Doucoure has signed a four-year contract with the Toffees.
"Abdoulaye Doucoure has completed a transfer to Everton from Watford for an undisclosed fee, signing a three-year deal until the end of June 2023 with a Club option for a fourth season," Everton announced in a statement released on the club's official website on Tuesday.
"The French midfielder, 27, becomes the Blues' third major signing in four days following the arrivals of Brazil international Allan and Colombia playmaker James Rodriguez, both 29."
Ready for the Royal Blue. #BienvenueDoucouré@abdoudoucoure16pic.twitter.com/8Y9y9sUcok
— Everton (@Everton) September 8, 2020
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