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Purpose Opecoelids, a diverse group of digenean trematodes, pose a taxonomic challenge due to morphological similarities. The genus Genitocotyle is little studied with only five recognized species. Recent phylogenetic advancements have refined Opecoelidae classification, recognizing 15 subfamilies, including opecoelins, distinguished by the absence of a cirrus sac and a canalicular seminal receptacle. Within this subfamily, the genus Genitocotyle remains understudied, with only five recognized species. The aim of this study is to describe a new species and to clarify a long-standing taxonomic ambiguity. Methods Two historical specimens of Genitocotyle from Apogon imberbis off Madeira, preserved in the Natural History Museum, London, were examined alongside newly collected specimens from Algerian waters. Comparative morphological analyses, including illustrations and morphometric measurements, were conducted. Results Genitocotyle necromnemos n. sp. is described, differing from its congeners by its larger body size, the more numerous eggs, the extended post-testicular region, the seminal vesicle restricted to the forebody, the extension of the vitellarium and the shape of the ovary. This new species designation clarifies the identity of some museum specimens, undescribed since their collection in 1951. Conclusion This study highlights the importance of museum collections for the clarification of taxonomic uncertainties and the expansion of knowledge about the biodiversity of trematodes. The result contribute the systematics of opecoelines and emphasise the ecological and biogeographical importance of Genitocotyle spp. in marine fish hosts.