We evaluate the effectiveness of the MERCOSUR's network of protected areas in representing South America's ecoregions. The region contains 1,219 non-overlapping protected areas covering nearly 2,000,000 km2. Fifty percent of the reserves are <100 km2 and 75% <1,000 km2. Less than a half of the 75 ecoregions in the MERCOSUR have at least 10% of their area within protected areas, and only 13 when just reserves in IUCN categories I–IV are considered. In general, forests are better represented than other biomes. At the national level the network of protected areas in Uruguay is the least developed in the region, with those of Bolivia and Chile the most developed. For 10% of each ecoregion to be protected at least another 500,000 km2 would have to be incorporated into the network. Such expansion would be more efficient if conservation priorities are identified using a regional approach. This is of particular relevance for the cost-efficient protection of the 20 ecoregions that are shared by two or more countries. While only c. 20% of the ecoregions found in Brazil are shared with other countries, >75% of the ecoregions in Bolivia, c. 70% in Argentina, >60% in Chile, and all the ecoregions in Paraguay and Uruguay are shared with other countries. Overall, although it currently covers 14% of the region, the network of protected areas of the MERCOSUR still performs poorly in protecting its ecoregions.