The ant genus Lenomyrmex was recently discovered and described from mid to high elevation rainforests in southern Central and northwestern South America. Lenomyrmex currently consists of six described species, which are only rarely collected. Here, we add a new species, Lenomyrmex hoelldobleri sp. n., which was discovered in a stomach content sample of the dendrobatid frog, Oophaga sylvatica, from northwestern Ecuador. Lenomyrmex hoelldobleri can be distinguished from other species in the genus by the presence of a well-developed petiolar node, whereas in all other species the node of the petiole is ill-defined. In addition to the shape of the petiolar node, L. hoelldobleri can be distinguished from the morphologically similar L. costatus by (i) the presence of the metanotal suture, (ii) the direction of the striae on dorsum of propodeum (concentrically transverse in L. hoelldobleri, longitudinal in L. costatus), (iii) the finely striate dorsum of postpetiole, (iv) its larger size, and (v) distinctly darker coloration.…