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Placida cremoniana (Trinchese, 1892) is a sacoglossan heterobranch sea slug of striking coloration, with a reported range that has increased dramatically in recent years. Described from the Mediterranean in 1892, nominal conspecifics were reported from Japan in 1959, and since the 1990s, additional populations have been reported from across the tropical Atlantic, Pacific, and Indian oceans, plus temperate areas of Japan, Australia, and Europe. This sequence of events is consistent with a dramatic range expansion by one species due to human-mediated introductions, or dispersal from tropical into temperate areas induced by recent climate change. Alternatively, the expanding range of this nominal species could, instead, reflect the gradual discovery of previously unsampled members of a global species complex. These hypotheses were tested using a combination of molecular data from three loci (COI, 16S, and H3), as well as radular morphology and external color patterns, from specimens sampled from most of the reported range. Molecular species delimitation recovered P. cremoniana as a complex of four genetically divergent species, further distinguished by consistent differences in head coloration and radular morphology. As the Mediterranean species retains the name P. cremoniana, the three new species are described here, including an Eastern Pacific endemic and two species that are sympatric across large areas of the Central and Western Pacific. Photographic records and literature reports suggest that additional species may occur in the Indian Ocean, tropical Australia, and Caribbean.