Berghuis agreed personal terms with Watford last week but the transfer was held up while the clubs negotiated a fee.
Press Association Sport understands AZ have now accepted a bid close to £4.6million for the player and that Berghuis will arrive in London on Friday to undergo a medical.
The 23-year-old forward was AZ's star player last season, scoring 11 goals in 22 league appearances, and he also attracted interest from Eredivisie champions PSV Eindhoven this summer.
The move will take the Hornets' summer signings into double figures after Jose Manuel Jurado, Miguel Britos, Sebastian Prodl, Giedrius Arlauskis, Matej Vydra, Jose Holebas, Etienne Capoue, Valon Behrami and Allan Nyom all penned permanent deals at Vicarage Road
Source : PA
Source: PA