Two new Geissleria species are described from Portugal based on their morphological and ultrastructural characteristics. Geissleria tagensis Novais et Ector sp. nov. has rhombic-elliptical valves with broadly rounded to slightly protracted apices and lacks an isolated pore. Geissleria lusitanica Novais et Ector sp. nov. has linear-elliptical to elliptical valves with slightly to markedly protracted ends and two isolated pores. The type analysis of similar species (Geissleria acceptata (Hustedt) Lange-Bertalot et Metzeltin, G. schoenfeldii (Hustedt) Lange-Bertalot et Metzeltin and Navicula modica Hustedt) allowed their better characterization and illustration, supporting the delimitation of the new Portuguese species. A new taxon present in the type material of G. schoenfeldii, G. hinziae Novais et Ector is also described. Three new combinations are also proposed: Geissleria capitata (H. Kobayasi in Kobayasi et Ando) Ector et Novais comb. nov., G. modica (Hustedt) C.E. Wetzel, Novais et Ector comb. nov. and G. taishanica (Y.Q. Guo et S.Q. Xie) Ector et Novais comb. nov. A checklist of the species and a thorough survey of the literature available on Geissleria are provided.