The Brazilian Atlantic Forest is one of the most endangered and diverse biomes on the planet. The region houses thousands of endemic species. Among these is Xenolycus Ferreira and Silveira, 2020, a monospecific net-winged beetle (Lycidae). Xenolycus possesses the features found in other lycids known or suspected to be affected by the paedomorphosis syndrome, a phenomenon in which adult individuals retain juvenile characteristics. Xenolycus was originally assigned to the tribe Calopterini (Lycinae), a Neotropical group containing several paedomorphic taxa. We generated genomic data for Xenolycus to test its placement within Lycidae. We analyzed our datasets using 2,260 Elateroidea-specific orthologs ("Elaterbaits") at nucleotide and amino acid levels, using a maximum likelihood and multispecies coalescent approaches. Our results refuted that Xenolycus is a member of Calopterini. Instead, we recovered it in a basal position within Lycidae, sister to the Calochrominae. Based on the results of the phylogenomic analyses, we created Xenolycinae subfam. nov. to accommodate Xenolycus, increasing the number of subfamilies within Lycidae to 8, and indicating another hypothesized origin of the paedomorphosis syndrome in the family. We discuss the implications of our findings for the classification of Lycidae, diagnosing the new subfamily and providing a thorough comparison of Xenolycus with the closely related Calochrominae.