Social monogamy with genetic polygamy is a frequent mating system in birds. In Passeriformes, extra-pair paternity (EPP) was reported in 86% of the studied species, with high variation in occurrence and frequency within and between species. In the highly diverse Neotropical region, the genetic mating system is known for only 26 bird species. We present the first molecular data on the genetic breeding system of the Ringed Warbling-finch Poospiza torquata, a socially monogamous species of the tanagers family (Thraupidae) that inhabits arid and semi-arid regions of South America. We obtained data from 92 nestlings belonging to 35 nests during three consecutive breeding seasons (2008–2011). Paternity assignments were made using five microsatellites developed for other bird species, and calculated using CERVUS v. 3.0. We found that extra-pair males sired 20.65% of the offspring in 42.86% of the broods. The EPP level in this species was found to be close to the average level in Thraupidae, with reported values ranging between 8–50% and 15–63% for nestlings and broods with EPP, respectively. This wide interspecific variation points to the ecological factors as being very important in facilitating EPP among Thraupidae species.