Eccopsis razowskii Vargas (Lepidoptera: Tortricidae: Olethreutini) is a Neotropical micromoth native to the Atacama Desert with leaf-tying larvae that feed on Acacia macracantha Willd. (Fabaceae). The external morphology of its last instar and that of its pupa are described and illustrated for the first time with the aid of light and scanning electron microscopy. The morphology of each of these life stages is highly similar to that of the only sympatric congeneric, the pest species E. galapagana Razowski & Landry, the larvae of which feed on Prosopis spp. (Fabaceae). The pupae of the two species cannot be separated based on morphology; they can only be accurately separated based on the form of the mandible. The findings are discussed in terms of their significance for the taxonomy and systematics of Eccopsis. In addition, a comparison with the two other Tortricidae associated with A. macracantha in the Atacama Desert is provided.