Two Craig Dawson headers either side of Hal Robson-Kanu pouncing on David Ospina spilling a cross were too much for the visitors, whose goal came from Alexis Sanchez.
As Arsenal fans battled out over Arsene Wenger’s future with aerial protests, the Gunners found themselves utterly outclassed by a powerful, organised Albion side.
Further up the table, Gary Cahill's 87th-minute header saw Chelsea beat Stoke City and open up a 13-point gap at the top of the standings ahead of Tottenham and Manchester City playing on Sunday.
EPL | The Premier League table after 29 matches
In the battle against relegation, Crystal Palace sealed a third straight win despite not recording a shot on target against Watford.
Troy Deeney's own goal was enough to settle a dire game at Selhurst Park and the Eagles move four points clear of 18th-placed Hull City,
The Tigers, meanwhile, suffered a 4-0 thumping at the hands of Everton with contract rebel Romelu Lukaku netting twice after Tom Huddlestone was sent of.
Everton are now level on points with the beleaguered Gunners, albeit having played two more games.
West Ham failed to recover from two early Leicester City goals to go down 3-2 at the London Stadium as their hopes of a top-10 finish were hit again.
Swansea City lost 2-0 at Bournemouth to remain in 17th, while the Cherries move up to 11th.
Elsewhere, Sunderland wasted multiple chances in a 0-0 stalemate with Burnley to remain rooted to the bottom of the table.