England Under 21 striker Theo Walcott made his first start for Arsenal after his two goals against Germany this week.
Arsenal defender Toure made it 1-0 for the Gunners, 32 minutes into the match. Henry doubled the lead in the 43rd minute.
Adebayor finished the scoring and made it 3-0 to Arsenal in the 67th minute, after a great foreplay by Cesc Fabregas and Henry.
Arsenal go fourth on the Premiership table, while Watford stay second bottom.
Lehmann, Djourou, Gallas, Toure, Hoyte, Walcott, Fabregas, Silva, Rosicky, Adebayor, Henry. Subs: Almunia, Van Persie, Hleb, Song Billong, Clichy.
Foster, Doyley, DeMerit, Shittu, Stewart, Smith, Mahon, Francis, Bouazza, King, Young. Subs: Lee, Mackay, Henderson, Powell, Spring.