Said Benrahma has signed a four-year deal after completing a medical at the club ealier in the week.
The 22-year-old has so far played all his football in France, joining Nice as a teenager and making his senior debut at 18.
He spent last season on loan with second division French team LB Chateauroux, and scored ten goals in 33 games in all competitions.
In 2015 he earnt his first cap for Algeria, and leaves Nice after scoring three goals in 18 games.
Rasmus Ankersen, the Brentford co-director of football, said: "Said is a player with lots of potential."
"He has the pace and technical skills to cause many Championship defenders problems, and we are pleased that it was possible to sign him early in the window so he can now go with the rest of the team to the pre-season camp."
July 6, 2018