A new coccidian species (Protozoa: Apicomplexa: Eimeriidae) is described from a specimen of the Eurasian blackbird Turdus merula Linnaeus held for rehabilitation and reintroduction into the wild in a centre for research and recovery of wild animals in Quinta de Marim, Olhão, Portugal. Isospora lusitanensis n. sp. has subspherical to ovoidal oöcysts, measuring on average 26.4 × 23.4 μm, with smooth, bi-layered wall c.1.1 μm thick. Micropyle and oöcyst residuum are absent, but a polar granule is present. Sporocysts are ellipsoidal, measuring on average 16.0 × 10.9 µm. Stieda body is knob-like and sub-Stieda body is prominent and rounded. Sporocyst residuum is composed of scattered spherules. Sporozoites are vermiform, with one refractile body and a nucleus. The morphological and morphometric data for the new species were compared with those for species parasitising birds of the Muscicapidae, Turdidae, Timaliidae, Troglodytidae and Cinclidae, which are considered phylogenetically close. The original histograms of Isospora turdi Schwalbach, 1959 were redrawn for comparison with I. lusitanensis n. sp. and a linear regression of width against length of the oöcysts is presented for characterisation. This is the first isosporoid coccidian described from T. merula in mainland Portugal.