The French striker moved for an undisclosed fee on a three-year deal and plugs the gap left by Brazil international Richarlison, who joined Tottenham in this transfer window.
Everton's other main striker, Dominic Calvert-Lewin, is out injured and manager Frank Lampard has been playing without a recognized center forward this season.
Everton has two goals in three matches in the league. One was an own-goal.
Maupay, who joined Brighton from Brentford in 2019 and scored 26 goals in 102 games, lost his place up front at Brighton to Danny Welbeck.
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