The 19-year-old defender arrives for an undisclosed fee from Auxerre and has agreed a three-year contract at Griffin Park.
Bees manager Uwe Rosler told the club website: "I am delighted that we have been able to get another young prospect.
"Raphael is a player that has represented his country at almost all age groups. That Brentford can attract a player like this is very, very good news.
"Raphael is a man for the future, he is very good technically but he is also a defender that likes to defend.
"He has been educated tactically and technically, as you would expect from a player that has come through the French system.
"He will need time to adapt to a new life, both culturally and in football terms. He will train as part of the first-team group but is likely to play his first games for the development squad.
"We will give him all the time he needs to settle in. Hopefully he will adapt quickly and play for our first team."