Joe Rindl, BBC Sport
West Ham's opener, from Brighton's perspective, was sloppy.
The midfielder's first effort was blocked by Billy Gilmour but spun back into his path for an easy tap-in.
It meant West Ham had their lead, and were able to sit back and hit the Seagulls on the break.
And there was also a small penalty shout with the ball deemed to have accidentally struck West Ham's Vladimir Coufal's arm in the box with the Seagulls 3-1 down.
Referee Anthony Taylor waved play on. A few minutes later Brighton's unbeaten start came to an end.
Roberto De Zerbi's side face Newcastle at home in their next fixture in seven days time.