Substitute Gianluca Scamacca scored the winner against Anderlecht as West Ham recorded a third straight victory in the Europa Conference League.
Scamacca guided it into the bottom corner with a first-time strike on the turn, late in the second half.
West Ham goalkeeper Alphonse Areola was forced to make an excellent save to block Anderlecht striker Fabio Silva's header shortly afterwards.
The Hammers sit top of Group B with Anderlecht five points behind.
David Moyes' side controlled large spells of possession and had numerous chances at Lotto Park in Brussels.
There were nine changes from the side that beat Wolves in the Premier League in their last outing, with Michail Antonio and Said Benrahma among those given an opportunity.
Benrahma had the best chances in the first half when he had a shot blocked, before he was teed up by Jarrod Bowen, only to slice it over the bar from close range with the goal gaping.
Antonio and Bowen also came close, while on-loan Wolves striker Silva had an effort blocked in the box by his own team-mate Francis Amuzu for Anderlecht.
Goalkeeper Areola was called into action to deny Yari Verschaeren from long-range but was otherwise relatively untested until he pulled off that terrific save to deny Silva.
Moyes made changes with around 20 minutes remaining as West Ham searched for the winner - captain Declan Rice, midfielder Lucas Paqueta and striker Scamacca all came on.
The substitutions paid off as all three were involved in the goal as Paqueta set-up Scamacca to finish a silky team move.
West Ham manager Moyes told BT Sport: "We've started to get a squad of players we can chop and change when we need to. The players we brought on made a difference.
"Flynn Downes was the best player tonight, he did really well. He put a marker down."