First-half headers from Kevin Nolan and Carlton Cole, an earlier own goal from Luke Chambers, and Winston Reid's late tap-in gave West Ham victory at Forest.
West Ham opened the scoring when Chambers sliced Matt Taylor's cross into his own net before Nolan nodded home James Tomkins' headed cutback.
Cole made it 3-0 when nodding in from close range following Reid's pass.
Nottingham Forest reduced the deficit with Robbie Findley turning in Chris Cohen's cross before Reid made it safe.
It was an impressive display from the Hammers who are showing why they are heavily tipped for an instant return to the Premier League while Forest manager Steve McClaren will be keen to bolster his squad before Wednesday's transfer deadline.
It was also a third successive away win for West Ham, who had only taken 12 away points from their previous 29 away league outings.
West Ham, without Tottenham-bound midfielder Scott Parker, showed their intent early on when Julien Faubert snatched at a close-range opportunity after Cole had robbed Chambers of possession.
The Hammers went ahead in the 21st minute when Matt Taylor whipped the ball into the six-yard box, only for Chambers to fumble it into his own net.
Nolan used his head to double the advantage, James Tomkins nodding the ball into a throng of visiting players for the former Newcastle midfielder to net his second this term.
Forest twice went close with Ishmael Miller shooting into the net netting from the right before Rob Green denied Lewis McGugan following Chris Gunter's weighted cross.
Sam Allardyce's side went 3-0 up in the 32nd minute when Cole nodded home Reid's pacey cross before Nolan wasted a fabulous chance to add a fourth when he blasted wide.
Forest introduced summer signings Andy Reid and Matt Derbyshire after the hour and the change soon paid dividends when Reid and Joel Lynch combined to put a ball across the box to Findley who slotted home.
West Ham still had time to add a fourth in the 77th minute when Reid latched on to Taylor's pinpoint cross to slip the ball beyond Lee Camp.