The 40-year-old retired from playing in 2011 following spells with the likes of Blackburn Rovers, Coventry City and Everton.
Carsley's previous coaching experience has seen him work for Coventry and Sheffield United.
Brentford academy director Ose Aibangee told the club's official website: "Lee brings with him the experience of playing more than 450 league games. That is knowledge he can pass on to our Under-21 players, the ones who we want to be knocking on the door of the first team.
"We believe we have a group of players that are capable of playing in the Championship and more coming up in the younger age groups. They will be able to use what Lee has learnt to enable them to take the next step."
Carsley's playing career also included making 40 appearances for the Republic of Ireland between 1997 and 2008.