Molecular phylogenetic analyses based on numerous plant accessions have shown that Cucumis comprises 25 species in Asia, the Malesian region, and Australia, rather than just two as traditionally thought. Among the 25 species several are new, and here we describe four from tropical Australia. The new species C. costatus, C. queenslandicus, C. umbellatus, and C. variabilis are illustrated, their distributions are mapped based on 7–50 collections per species, and information is provided on habitats and conservation status. We also validate C. althaeoides comb. nov. and C. argenteus comb. nov., update the description of C. picrocarpus, and provide a key to the 11 native and naturalized species of Cucumis occurring in Australia and eastern Malesia. The Australasian species diversity of Cucumis, a genus that until recently was held to be essentially African, is of interest also because Australia harbours the sister species to the commercially important melon, C. melo.