Ndong's switch to the Stade du Moustoir was announced on Saturday, with the 20-year-old signing a four-and-a-half-year deal.
"I think Lorient is the best club for me in my progression," Ndong, who is set to represent Gabon in the Africa Cup of Nations later this month, told his new club's official website.
"I come from a very good club in Tunisia. FC Lorient practice the beautiful game with a team of young people.
"It is important to start my career in Europe. Now I have to have the opportunity to play. The place is not won.
"This is my first experience in Europe and I think Lorient will make me progress. I'm here to find out."
Ndong revealed countryman Bruno Ecuele Manga, a former Lorient skipper who now plays for Cardiff City, had influenced his decision.
"Yes, it helped a lot. He was captain here and he talked a lot about it to me," Ndong added.
"He said it was a good club. He loved his time here and was very loved. Bruno loves FC Lorient and thinks it is the best choice for me."
Lorient coach Sylvain Ripoll said: "He is a complete player who can play offensive or defensive midfielder, a sharp player with lots of character in personality and in the game."
