Although the Indo-Eastern Mediterranean giant water bug Lethocerus patruelis (Stål, 1854) is Europe’s largest hemipteran and aquatic insect, its distribution in Greece has remained poorly understood. Only a handful of records exists from the Ionian Islands, Macedonia, Thrace, and isolated observations in Preveza (Epirus), Achaia (north-western Peloponnese) and the islands of Rhodes and Thasos. Using records from personal observations, biomonitoring data, online arthropod identification groups and museum collections, we expand the distribution knowledge of this species to Aetolia-Acarnania, Thessaly, southern Peloponnese, Samothraki, North Sporades and the Cyclades. We also include a large number of new localities in areas within its previously known range.