Here we argue that a faster and more cost-effective way to reduce deforestation in the Brazilian Amazon would be the immediate allocation of ca. 70 million hectares of still undesignated public forestlands to conservation and social use. Currently, this massive block of forests is not under effective supervision by a designated public agency, increasing the risk of continued land grabbing and predatory use. These undesignated public lands accounted for 25% of recent deforestation, emitting 200 million tons of CO2. Under the current scenario of deforestation growth in the region, the immediate allocation of undesignated forestlands to conservation or social use by the government will reduce the availability of unsupervised public land, increase forest protection and, therefore, decrease deforestation and carbon emissions. Additionally, the action would send an unmistakable sign to the international community of Brazil's will to increase governance of its large share of the Amazon forest.