Nicolas Nkoulou hopes to help Lyon "achieve great things" after joining them from Marseille.
With reports linking Lyon defender Samuel Umtiti with a switch to Barcelona after Euro 2016, Les Gones have moved to strengthen their back line with the capture of Nkoulou, 26.
The Cameroon international was unveiled at a news conference on Wednesday where he said he is looking forward to getting going at his new club.
"I am here because OL is a club which can help me continue to grow. It's a real pleasure to be here," he said. "I will wear this shirt with a lot of desire and I will work to achieve great things.
"I spent five good years at OM, I was out of contract and it was time to turn the page.''
Nkoulou began his career at Monaco in 2008 before joining Marseille in 2011, where he won the Coupe de la Ligue in 2012.
He will provide reinforcements for Lyon's defence, with Bakary Kone having left for Malaga and Lindsay Rose linked with Lorient. Umtiti has been linked with Barca and Lyon coach Bruno Genesio has admitted it is "difficult to resist" if he wishes to move to the Camp Nou.
Bienvenue Nicolas Nkoulou !!
- Olympique Lyonnais (@OL) June 29, 2016
Lyon president Jean-Michel Aulas said: "Nicolas Nkoulou was a priority of the coach and has signed for four years. We are very happy.''
Aulas' advisor Bernard Lacombe added: "Nicolas is a player of great quality. We have wanted him since he was at Monaco.''
Lyon have also been linked with River Plate's 20-year-old defender Emanuel Mammana.
Nkoulou will take his favoured No. 3 shirt, vacated by Henri Bedimo's move in the other direction to Marseille last week.
"The No. 3? It is a wink to those close to me, because I am the youngest of three children,'' he said of his attachment to the shirt number he also wears at international level.