Aston Villa will be the first opponent, with the two teams competing for the Graham Taylor Trophy on July 29.
Taylor, who died on January 12 aged 72, managed both Watford and Villa before going on to take charge of England and Wolves. He then returned for second spells with the Hornets and Villa before his retirement from management.
A statement on Watford's official website said: "Watford will host Aston Villa on Saturday July 29 at Vicarage Road (3pm kick-off), in a special memorial matchday for the late Graham Taylor OBE.
"At the request of his family, profits from the friendly will be split evenly between two charities Graham had a close connection to - the National Memorial Arboretum and the Hornets' self-financing registered charity, the Watford FC Community Sports and Education Trust."