A comparative study on forests and grasslands in three ecoregions (Humid Chaco, Espinal, and Paranaense) was conducted in two protected areas in northeastern Argentina: Iberá National Park and Mburucuyá National Park. The effects of habitat heterogeneity (vegetation cover) on terrestrial ant (Formicidae) assemblages were analyzed and compared. The habitat heterogeneity hypothesis—which predicts that when environmental structural complexity increases, the species richness will also increase—was tested. Two sites were selected in each ecoregion. A forest and a grassland were surveyed in each site, using unbaited pitfall traps from October 2013 to February 2014. Overall, 5,465 ants belonging to 37 species were collected, 32 of which were present in forests and 26 in grasslands. Solenopsis invicta Buren (Hymenoptera: Formicidae) and Pheidole radoszkowskii Mayr (Hymenoptera: Formicidae) were the most abundant species. Formicidae assemblages were compared in terms of abundance, species richness, and diversity between habitats and ecoregions. The results of this study support the habitat heterogeneity hypothesis, since we found a higher diversity of ants in more structured habitats (i.e., forest) in the three ecoregions analyzed. Also, the ant assemblages differed both between habitats and between ecoregions. Our findings provide the first assessment of terrestrial ant assemblages in natural habitats of the three ecoregions in Corrientes Province, Argentina.