The Morocco striker made a big impression at Irn-Bru First Division side Falkirk last season after he was signed on a free transfer. The 27-year-old scored 18 goals for Steven Pressley's side, including a brace against Rangers in a 3-2 victory in the League Cup.
Kilmarnock, and fellow SPL side St Johnstone, were keen to give the former Bergerac forward a chance to play in the top flight, but El Alagui settled on a move to England's third tier and Brentford boss Uwe Rosler said he was happy to secure his signature. "He has a good goalscoring record and has shown consistency over the past two years," he said.
"I see him as a late developer, he is 27 now and there is more to come from him. We expect goals from him - he can add something to our squad."