Toumani Diagouraga has joined Brentford on a three-year deal for an undisclosed fee from Peterborough.
The 23-year-old midfielder spent the second half of last season on loan with the Bees where he made 20 appearances.
Brentford boss Andy Scott told his club's official website: "It is no secret that Toumani has been a long-term target of mine and I am absolutely delighted that I have at last secured his services.
"I feel that at his age and his level of experience he will be a very important player for us both now and in the future."
Diagouraga added: "I am really pleased that at long last I have been able to sign a contract with Brentford and that I can now concentrate on doing what I do best."