The César Award for Best Actress in a Supporting Role (French: César de la meilleure actrice dans un second rôle) 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.

With three wins (1991, 1993, 1999), Dominique Blanc holds the record of most César Award for Best Supporting Actress.

Eight actresses have won the César twice: Marie-France Pisier (1976, 1977), Nathalie Baye (1981, 1982), Suzanne Flon (1984, 1990), Annie Girardot (1996, 2002), Valérie Lemercier (1994, 2007), Julie Depardieu (2004, 2008), Anne Alvaro (2001, 2011) and Karin Viard (2003, 2019).

They are Emmanuelle Béart, Charlotte Gainsbourg, Dominique Blanc, Anne Brochet, Karin Viard, Sandrine Kiberlain, Emmanuelle Devos, Cécile de France, Marion Cotillard, Sylvie Testud, Emilie Dequenne, Sara Forestier and Adèle Haenel.

Nevertheless, Virna Lisi's and Carmen Maura's roles included some parts of Italian-language and Spanish-language, respectively.

For example, the César for Best Supporting Actress of 2010 was awarded on February 27, 2010 for a performance in a film released between January 1, 2009 and December 31, 2009.