The Hornets beat Stoke 2-0 last weekend, and have now built on that with another win today, pushing them into ninth place with 16 points.
Quique Sanchez Flores' side took the lead in the first half when Andy Carroll's dallying in his own box gave possession to Ikechi Anya, and his cross was bundled over the line by striker Odion Ighalo - though it may well go down as an own goal from Aaron Cresswell.
Ighalo then made it two after half time, slamming the ball past Hammers goalkeeper Adrian to give West Ham - who have been so impressive so far this season - an uphill task.
Their dismal afternoon was then compounded when defender James Collins was sent off for a foul on the outstanding Ighalo late on, sealing the win for Watford.
West Ham drop down to fifth after this defeat, with Slaven Bilic's side unable to build on their 2-1 win over Chelsea from last weekend.