Social parasites exploit the colony resources of social species to secure their own survival and reproduction. Social parasites are frequently studied as models for conflict and cooperation as well as for speciation. The eusocial Hymenoptera harbor a diverse array of socially parasitic species with idiosyncratic life history strategies, but it is probably in the ants where social parasites are most speciose and have evolved the highest degrees of morphological and behavioral specialization. In the fungus-growing ants, a total of five obligate social parasites are known: four species are parasites of leaf-cutting ants and one species parasitizes a primitive fungus-growing ant species in the genus Mycocepurus. Here we describe a new species of socially parasitic leaf-cutting ant, Acromyrmex charruanus sp. nov., from Uruguay, and we report initial observations on the parasite’s life history as well as on the morphological and behavioral adaptations related to the inquiline syndrome. Our observations suggest that Acromyrmex charruanus is an obligate inquiline social parasite of the thatch-mound-building, leaf-cutting ant Acromyrmex heyeri. Acromyrmex charruanus appears to be tolerant of the host, producing sexual offspring in the presence of the A. heyeri host queen. Queens of A. charruanus appear to reproduce semelparously and sexual offspring are produced during the austral fall (February), which differs significantly from the mating biology of the host species, which reproduces during the southern-hemisphere spring (October–December). We suggest that the diametrically opposed mating seasons of parasite and host might be adaptive, allowing the parasite to avoid competition for resources with the host sexual brood. The morphological and behavioral adaptations of A. charruanus accord with characters observed to arise early during the evolution of other ant inquiline parasite species, and so far we have no evidence for the existence of a worker caste in A. charruanus. Further field studies and behavioral experiments need to confirm our first observations and explore A. charruanus’s behavioral ecology, evolution, and life history in more detail.