The César Award for Best Actress in a Supporting Role is one of the César Awards, presented annually by the Académie des Arts et Techniques du Cinéma to recognize the outstanding performance in a supporting role of an actress who has worked within the French film industry during the year preceding the ceremony. Nominees and winner are selected via a run-off voting by all the members of the Académie.

The César Award is the national film award of France. It is delivered in the Nuit des César ceremony and was first awarded in 1976. The nominations are selected by the members of twelve categories of filmmaking professionals and supported by the French Ministry of Culture. The nationally televised award ceremony is held in Paris each year in February. The exact location has changed over the years. It is an initiative of the Académie des Arts et Techniques du Cinéma, which was founded in 1975.