A new benthic freshwater diatom, Fragilaria rinoi Almeida & C.Delgado sp. nov., is described from river periphyton samples in Portugal. Fragilaria rinoi sp. nov. is illustrated and discussed based on populations collected from the Vouga, Mondego and Lis river basins in central Portugal and compared with the type material of Fragilaria vaucheriae (Kütz.) J.B.Petersen. The morphological features of the new diatom species are documented through light and scanning electron micrographs, including a comparative analysis with related species of the genus (F. candidagilae Almeida, C.Delgado, Novais & S.Blanco, F. intermedia Grunow in Van Heurck, F. neointermedia Tuji & D.M.Williams, F. recapitellata Lange-Bert. & Metzeltin, F. perminuta (Grunow) Lange-Bert., F. vaucheriae and F. microvaucheriae C.E.Wetzel & Ector). Fragilaria rinoi sp. nov. is characterized by solitary cells without spines, lanceolate valves with slightly rostrate apices, a narrow, linear axial area, and a large, unilateral central area.
Fragilaria rinoi sp. nov. may be confused with F. microvaucheriae in terms of length, striae density and outline, although a morphometric analysis revealed that F. rinoi sp. nov. is significantly wider. Fragilaria rinoi sp. nov. is present in rivers with high dissolved oxygen concentrations, medium to high conductivity, neutral to slightly alkaline pH and high mean values of nitrates and ammonium.