Duje Caleta-Car is having his Southampton medical after they agreed a fee of around £8m with Marseille.
The Croatian was on Southampton's deadline day centre-back list as a replacement for their defensive departures.
Jack Stephens has joined Bournemouth on loan and Jan Bednarek is Aston Villa bound, having previously been lined up by West Ham.
Caleta-Car is set to sign a four-year deal at St Mary's.
Southampton have enjoyed a solid start to their Premier League campaign after a busy summer.
Ralph Hasenhuttl's side climbed above Chelsea into seventh after beating the west London side 2-1 at St Mary's earlier this week.