Maelys Mpome, 22, is heading to the Seagulls after a season in West London.
Mpome made eight appearances for the treble-winners last season and has been capped by France at junior levels up to under-23s.
In the official announcement of the deal, Albion head coach Dario Vidosic said: “We’re pleased to have Maelys with us. She’s had some exposure to the WSL already and we feel she can really develop with us.
“She’s a versatile defender with a lot of good attributes and we’re looking forward to working with her.”
Mpome had four years at Montpellier having started her career with her local French second-tier club VGA Saint-Maur in Paris.
Albion have already added defenders Manuel Vanegas and Moeka Minami to their squad in what has been a summer of change at Lancing.
They have a pre-season camp in Spain amid growing excitement about an imminent update on their plans for a purpose-built stadium.
WSL action gets under way on Sunday, September 7 when Aston Villa head to the Broadfield Stadium (midday).