The 22-year-old made 40 appearances for Lille last season, helping them clinch the Ligue 1 title in May.
Soumare helped France reach the quarter-finals of the European Under-21 Championship in May and said he is “proud and honoured” to join the Foxes.
“Leicester is a very ambitious club,” he said.
“When they spoke to me about their plans and what they wanted from me, I knew straight away I wanted to come.”
Soumare made 104 appearances in four seasons with Lille after joining them from Paris St-Germain, where he featured as a youth player.
