The midfielder was Ranieri's final signing at the Foxes when he joined from Genk for £17million in January 2017.
Ndidi has made 188 appearances for the Foxes, helping them win the FA Cup last season, and thanked Ranieri for having faith in him.
“I'm very grateful to him because he gave me the opportunity to come to Leicester and play in the Premier League for the first time," Ndidi said.
“That was a really important move in my career, so I'll always remember it. Joining Leicester City has been fantastic for me. It's a great club."
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