Cássia Rejane Eller (Portuguese: /ˈkasjɐ ʁeˈʒɐni ˈɛleʁ/) (December 10, 1962 – December 29, 2001) was a Brazilian singer, composer, and multi-instrumentalist, regarded as one of the greatest representatives of Brazilian rock in the 1990s.

She came to prominence in the early 1990s and performed a mix of rock and MPB. Eller released five studio albums in her lifetime: Cássia Eller (1990), O Marginal (1992), Cássia Eller (1994), Veneno AntiMonotonia (1997) and Com Você... Meu Mundo Ficaria Completo (1999). Her sixth studio album, Dez de Dezembro (2002), was released posthumously. Eller's most successful album was Acústico MTV – Cássia Eller (2001), selling over 1 million copies. She was ranked as the 18th-greatest vocalist and 40th-greatest Brazilian musician by Rolling Stone Brasil. On December 29, 2001, Eller died at the age of 39 of a heart attack caused by a malformation of her heart.