WATCH the Bundesliga LIVE on beIN SPORTS | via Foxtel, Kayo, Fetch & beIN SPORTS CONNECT
Sommer's eight-and-a-half-year stint with Gladbach came to an end on Thursday when joining Bayern Munich for a fee reported to be in the region of €8million.
Gladbach have turned to fellow Switzerland international Omlin, who made 74 appearances for Montpellier after joining from Basel in August 2020.
The 29-year-old, who earned the most recent of his four Swiss caps last June, has signed a deal at Borussia-Park that runs through until July 2025.
"We believe we have found a very good solution by signing Jonas Omlin," Gladbach sporting director Roland Virkus said.
"He is a very reliable goalkeeper who exudes confidence, is strong on his line and is good with the ball at his feet."
Omlin appeared in 14 of Montpellier's 19 games this season, keeping three clean sheets.