A new coccidian species of the genus Caryospora Léger, 1904 (Apicomplexa, Eimeriidae) is described based on material from the Guanabara spotted night snake Siphlophis pulcher (Raddi) (Reptilia: Dipsadidae) in a coastal area of the Atlantic Forest in Ilha Grande Island, state of Rio de Janeiro, southeastern Brazil. Caryospora ceadsensis n. sp. possesses spheroidal to slightly subspheroidal oöcysts measuring 17-24 × 17-24 (22.1 × 22.0) µm, with a c.1.3 µm thick bi-layered wall (inner layer smooth, outer layer slightly striated), length/width (L/W) ratio of 1.0-1.1 (1.0) and a highly refractive polar granule. Sporocysts are ellipsoidal, 11-17 × 10-13 (14.7 × 11.2) µm, with a L/W ratio of 1.0-1.5 (1.3), with a nipple-like Stieda body (1.1 µm high and 2.9 µm wide) and a large, bubble-shaped sub-Stieda body 1.7 µm high and 3.8 µm wide (1.0-2.0 × 3.0-4.5 µm). Sporocyst residuum composed of granules of differing sizes. Sporozoites with striations. This is the sixth record of a species of Caryospora in snakes of the family Dipsadidae in Brazil.