The target species of this work is a large tritonid nudibranch, relatively common in Galician waters (north-western Spain), where it has been regularly observed since the year 2000. To date, this species has been misidentified as Tritonia hombergii, a species from northern latitudes, because of its remarkable size. On the other hand, the external anatomy of the specimens from north-western Spain resembles that of Marionia blainvillea, a species common in the Mediterranean. To untangle the real taxonomic identity of the species, a detailed anatomical study was performed, revealing several morphological differences that has led us to consider the Galician specimen as new to science. A phylogenetic analysis was conducted for both 16S and COI genes to try to establish its systematic position in relation with other taxa. Marionia gemmii, sp. nov. differs from all known Tritoniidae in the size, colour pattern, number of velar processes, presence of stomach plates, radular formula, the shape of spawning masses and even in its feeding habits. The phylogenetic results support the comprehensive morphological analysis and also make it clear that most tritoniid genera currently recognised are not monophyletic and that a major systematic revision of the family is needed. http://zoobank.org/urn:lsid:zoobank.org:pub:91332872-9020-4D06-8B1C-2E7654D54E1A