Carlo Ancelotti's renovations at Everton continue.
"When I heard the names of Allan and James joining Everton it was even more exciting to come. I did everything in my power to come here as soon as possible," Doucoure said.
"It has been a long time (negotiating the transfer this summer) and I am happy to come here in this exciting moment."
Doucoure joined Watford from Rennes in 2016 and quickly established himself in the first team after a season-long loan at Granada. He made 129 Premier League appearances before last season's relegation and fulfilled defensive and attacking duties in the heart of the Hornets' XI.
He joins a midfield that struggled over the 2019-20 Premier League campaign. Jean-Philippe Gbamin and Andre Gomes dealt with injury problems, Fabian Delph didn't consistently provide the sturdiness required in the middle of the park, and Gylfi Sigurdsson - the main creative outlet of the ensemble - scored twice and assisted only three times. Academy product Tom Davies has also not fully delivered on the promise he displayed during his 2016-17 breakout term.
Everton boss Ancelotti now expects Allan, Colombian playmaker James, and Doucoure to fix that part of his lineup.
The club begins its 2020-21 season with Sunday's trip to Tottenham Hotspur.