Sinemurian ostracodes from the Lusitanian Basin (western Portugal, Iberian Peninsula) were studied using twelve samples from the marl-limestone succession of the Coimbra Formation at S. Pedro de Moel, north of Lisbon. The detailed taxonomical study identified 11 genera and 14 species, among which 3 are new species: Phraterfabanella boomeri Cabral and Colin n. sp., Klieana? coimbraensis Cabral and Colin n. sp. and "Klinglerella" roselinae Cabral and Colin n. sp. The recognized ostracode species are particularly similar to those described from the uppermostHettangian-Sinemurian offshore Southwest Ireland (Fastnet Basin) and France (Normandy, Central France and Quercy). Overall, the assemblages include four brackish species (one clearly dominant) and ten marine species. The paleoecological analysis of the assemblages allowed a detailed paleoenvironmental reconstruction, reflecting frequent variations from brackish to restricted marine to slightly more open marine settings, which is supported by sedimentological and other paleontological data.