The genus Tethya represents a large cosmopolitan group of 81 recognized and a significant number of additional, still undescribed sponge species. The phylogenetic relationships within this old taxon are almost unknown. This is represented by the fact that several species are regarded as cosmopolitan. However, some of them might represent groups of closely related but distinct species. Using molecular and morphological characters, we aimed at a fundamental phylogenetic study for the genus Tethya, evaluating an existing morphological matrix and the cytochrome oxidase subunit I (COI) as a molecular marker. Intending to create a basic phylogenetic framework for the genus we selected a number of species from main biogeographic regions covered by Tethya species: from the Mediterranean Sea (T. citrina and T. aurantium), the northern European seas (T. citrina, T. norvegica and T. hibernica), the western Atlantic (T. actinia), the eastern Pacific (T. californiana), the Indo-Pacific (T. seychellensis) as well as three species of unknown biogeographical origin, originally described from aquarium habitats (T. wilhelma, T. gracilis and T. minuta). Both the morphological matrix, consisting of 29 characters and 131 character states, as well as the molecular matrices (COI nucleotide sequences of 658 bp as well as deduced amino acid sequence of 219 aa), resolved major biogeographical subgroups within the genus. However, the nucleotide based matrices revealed a higher resolution. Two major branches seem to have diverged early: an Indo-Pacific (T. seychellensis, T. wilhelma, T. minuta) and a Pan-European clade (T. citrina, T. norvegica, T. hibernica). For T. aurantium as well as a North-American group (T. californiana, T. actinia and T. minuta) our results are not conclusive. The morphological character set will have to be re-evaluated to gain a higher phylogenetic resolution. The addition of more species to our basal phylogenetic framework will allow analysing the evolution of the various biogeographical subgroups in the genus Tethya.