A new coccidian species of the genus Isospora Schneider 1881 (Apicomplexa, Eimeriidae) is reported from a single specimen of chelid turtle, Platemys platycephala (Schneider, 1792), obtained in a flooded area of the Amazonian municipality of Altamira, in Pará State, northern Brazil. The oöcysts of Isospora platemysi n. sp. are subspheroidal, 19.0-24.0 × 22.5-18.5 (22.6 × 20.8) µm in size [length/width ratio 1.0-1.1 (1.1)], with a colourless wall, 1.0-1.9 (1.4) wide, with two smooth layers; polar granule is present, oöcyst residuum and micropyle are both absent. The sporocyst is ovoidal, 10.0-12.1 × 8.0-10.0 (11.0 × 9.1) µm [length/width ratio 1.1-1.3 (1.2), with a knob-like Stieda body. Sub-Stieda body is present, rounded irregular-trapezoidal; para-Stieda body is absent. Sporocyst residuum is dispersed and composed of granules of differing sizes. This is only the fifth record of Isospora in chelonians, and the first report of a coccidian parasite of the genus Platemys.