During a recent survey of leaf-mining microlepidoptera in the Cerrado biome, mines of an undescribed Porphyrosela Braun (Lepidoptera: Gracillariidae: Lithocolletinae) were found associated with the forage peanut, Arachis pintoi Krapov. W.C. Greg. (Fabaceae). Consequently, adults, immature stages and the leaf mine of Porphyrosela arachisella sp. nov. are herein described based on light and scanning electron microscopy. A preliminary analysis of DNA barcode sequences including putative members of other lithocolletine species and all BINs (Barcode Index Numbers) available for Porphyrosela supports P. arachisella as an independent cluster, with 8 to 11% divergence. Its nearest neighbour was the cluster formed by three BINs (BOLD: ADT2137, BOLD: AAG1161 and BOLD: ADU9985) that includes specimens from Australia, Vietnam and Bangladesh. This is the first report of a lithocolletine gracillariid for Brazil, and the third species recognized for the genus in the Neotropical region.