The Spaniard has penned terms to 2022.
“I'm very happy to have signed. Newcastle United is a big club, and I'm very happy to be here," said Lejeune.
“When you have a manager like Rafa Benitez who wants you, you are very happy to come. Since I've been in touch with the manager, it was an easy decision for me to come here."
Manager Rafa Benitez added: “I would like to welcome Florian to Newcastle United. He is a player that has done very well this year in Spain. I have known about him for a while, since he began playing for Villarreal.
"He has progressed a lot as a player in the last few years and I know he will give us something different at the back - he is good with both feet, he's big and he's good in the air also.
"If he settles well into English football then I am sure he will be a success and will give us a lot of competition in this area."