A collection of 32 specimens of the genus Plakortis (Demospongiae: Plakinidae) from Australia and the Indo-Pacific is here examined. Six species are described, four of which are new to science. Plakortis lita has microrhabds, an irregular tangential ectosomal reticulation, a confused choanosomal skeleton and irregular diods and triods. Plakortis quasiamphiaster has quasiamphiasters (spined diods and triods), spheres and a skeleton with a distinct subectosomal region. Plakortis communis sp. nov. has diods and a tangential ectosomal reticulation with circular meshes; triods and spheres may be present or absent. Plakortis bergquistae sp. nov. has diods in two size-classes, the larger one up to 330–356 µm long, and large triods (actines up to 75–121 µm long). Plakortis fromontae sp. nov. has large diods (up to 220 µm long), rare triods and a double ectosomal reticulation. Plakortis hooperi sp. nov. has diods, triods, microrhabds and spheres, with a confused ectosomal skeleton, a thinly encrusting shape and cream colour. Detailed descriptions of external morphology, skeletal arrangement of the ectosome and spicules in SEM are essential for the taxonomy of Plakortis. The number of valid species in the genus is raised from 15 to 19. A key to Indo-Australian species of Plakortis is given.