Wallemme told officials he wanted to step down after the last game before the winter break against Bordeaux, Martel said in a statement.
"Laszlo Boloni's mission is to keep the team in Ligue 1. We need to focus on avoiding relegation," he told a press conference."Jean-Guy told me he was no longer able to give new impetus."
Boloni, 57, is a former Romania international. He has coached French clubs AS Nancy, Stade Rennes and Monaco and Belgium's Standard Liege.
Wallemme, a former Lens player, had been coach since 2008. He helped the Northerners reach the top flight in his first year and saved them from relegation last season. Lens are 19th in the 20-team standings on 16 points.
