Margaritasite is a yellow, caesium-bearing mineral in the carnotite group. Its chemical formula is (Cs, K, H<sub>3</sub>O)<sub>2</sub>(UO<sub>2</sub>)<sub>2</sub>V<sub>2</sub>O<sub>8</sub>÷H<sub>2</sub>O and its crystal system is monoclinic (space group P2<sub>1</sub>/a).
It was first described in 1982 from the Margaritas uranium deposit in the Peña Blanca district of the municipality of Aldama, in the Mexican state of Chihuahua.