Abstract Understanding the spatial distribution of species sheds light on the biogeographical history, offers clues to the drivers of biodiversity and helps guide conservation strategies. In southern South America, Andean, Chaco and Atlantic forests drastically decreased their coverage in the last decades mainly by changes in land use. Ants represent an excellent model for macro‐ecological studies that support evidence‐based management policies. Unfortunately, little is known about ant distribution, diversity patterns and drivers in the southernmost portion of these three biomes. We surveyed ground‐dwelling ants using baits and leaf‐litter sieves in 38 sites across these subtropical forests in Argentina. We analysed ant species richness, turnover and composition within and between forests, and whether spatial and climatic variables explain diversity patterns. We collected 161 species belonging to 38 genera. The Atlantic forest had the highest cumulative species richness (91 species), followed by the Chaco (60) and Andean (52) forests. Neither mean species richness nor beta diversity differed significantly between forests, though both indicators tended to increase from west to east. Seven spatial and climatic variables differentiated forests. Aridity index, and variables associated with seasonal rainfall, moderately influenced a tendency to increase species richness from west to east. We identified five shared species between the Andean and Atlantic forests, which are candidates to test biogeographic hypotheses. The high level of species replacement across and among forests resulted in unique assemblages (with at least 29 species that seem to be exclusive to one forest), indicating the importance of conserving the biodiversity of the three forests.