We studied body size and sexual dimorphism in the southernmost subspecies of the Burrowing Owl (Athene cunicularia cunicularia) at the Pampas of Argentina, and we compared our data with those from other subspecies in the Americas. A total of 58 individuals were captured and their coloration pattern (plumage and bare parts), body linear measurements (wing chord, standard tail length, tarsus length and width, forearm length, exposed culmen length, and hallux claw length), and body mass were recorded. In addition, we banded each individual and took a blood sample from 44 individuals for molecular sex determination. In general, the body size measurements reported for A. c. cunicularia in this study agreed with previous reports based on museum specimens for the same subspecies, and were near the upper range for the species. In addition, tarsi of A. c. cunicularia were large in comparison to those of other subspecies. Sexes differed little in size, with tarsus length, tail length, and wing chord being slightly greater in males, and culmen length and mass slightly greater in females. However, mean values of these measurements did not differ statistically between sexes. The southernmost subspecies of Burrowing Owls, A. c. cunicularia, has a larger body size than the North American and Caribbean subspecies, which suggests geographical variation in body size of this species throughout its distributional range.