Twenty-six-year-old Lejeune has put pen to paper on a five-year-deal with the Magpies, which will keep him at the club until the summer of 2022.
The Paris-born centre half is the second player to check in at St. James’ Park this summer, following the permanent arrival of winger Christian Atsu in May.
Lejeune began his career in his homeland with the Centre de Formation de Paris, before spending his formative years with Racing Club de France, Beziers, Sete and Agde.
He made the move to then-Ligue 2 club Istres in 2009, before moving on to Spanish side Villarreal two years later, where he progressed from the Yellow Submarine’s B team into their first team squad.
After a loan spell back in his homeland with Stade Brestois, Lejeune joined Girona, where he impressed, sealing a move to Manchester City in the summer of 2015.
A further loan at former club Girona followed, before the former France Under-20 international joined Eibar permanently in July 2016.
“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.”
Florian Lejeune's first interview as a Newcastle United player will be available to watch on NUFC TV later on Tuesday morning