The 23-year-old midfielder moves to Vicarage Road from Ligue 1 side Rennes.
The Hornets wrote on Twitter: "We're delighted here at #watfordfc to announce the signing of Abdoulaye Doucoure from Ligue 1 club Rennes
Welcome!"
Doucoure has signed a four-and-a-half-year deal, but has been immediately loaned to Granada in Spain for the rest of the season
The France Under-21 international will link up with Watford in the summer.
Watford chief executive Scott Duxbury said on the club's official website: "The opportunity to sign Abdoulaye Doucoure now was one we wanted to act upon, given our long-standing interest in the player.
"He is very much part of Watford's future plans, and in the time between now and pre-season he'll be experiencing high-level competitive football in Spain."
Source : PA
Source: PA