The 23-year-old returned from a second spell on the Toffees to the Nou Camp final summer season however has had a irritating season for the Catalan membership.
The Spaniard is the primary signing beneath the reign of newly appointed boss Javi Gracia.
Watford can pay the Spaniard’s wage throughout his time at Vicarage Street, plus a possible further cost of 1m euros.
Deulofeu socred 5 occasions for Everton, making 62 appearances in two totally different spells for the Merseyside membership.
The midfielder additionally spent the second half of final season on mortgage at Italian membership AC Milan.