The adult, pupa, larva and biology of Adenogasteria leguminivora Davis & Vargas gen. et sp. nov. (Lepidoptera: Gracillariidae: Ornixolinae) from Peru and Chile are described and illustrated. This micromoth was previously reported in the agronomic literature as a pest of Caesalpinia spinosa (Molina) (Fabaceae) in Peru, but its taxonomic status remained unknown until now. Its larvae feed on the seeds of three legume trees: Acacia macracantha, C. spinosa and Inga feuillei. Adenogasteria leguminivora superficially resembles other sympatric gracillariids but can be separated on key morphological features and host plant preferences. Eggs of A. leguminivora are laid on the fruit surface, from where the larva burrows until it reaches the cotyledons, which are mined by the two early (sap-feeding) instars and almost completely consumed by the last three (tissue-feeding) instars. Subsequently, the mature larva leaves the fruit and constructs an elongated silk cocoon ornamented with a few silk bubbles before pupation. A maximum likelihood analysis of DNA barcodes clustered the A. leguminivora sequences as a single independent phylogenetic unit within Ornixolinae, sister to the Chileoptilia + Philodoria clade, regardless of the host plant and geographic origin of the samples. The genetic divergence of A. leguminivora to the nearest neighbour in COI barcodes was >10%.