Secondary forests are increasing in the Brazilian Amazon and have been cited as an important mechanism for reducing net carbon emissions. However, our understanding of the contribution of secondary forests to the Amazonian carbon balance is incomplete, and it is unclear to what extent emissions from old‐growth deforestation have been offset by secondary forest growth. Using MapBiomas 3.1 and recently refined IPCC carbon sequestration estimates, we mapped the age and extent of secondary forests in the Brazilian Amazon and estimated their role in offsetting old‐growth deforestation emissions since 1985. We also assessed whether secondary forests in the Brazilian Amazon are growing in conditions favourable for carbon accumulation in relation to a suite of climatic, landscape and local factors. In 2017, the 129,361 km2 of secondary forest in the Brazilian Amazon stored 0.33 ± 0.05 billion Mg of above‐ground carbon but had offset just 9.37% of old‐growth emissions since 1985. However, we find that the majority of Brazilian secondary forests are situated in contexts that are less favourable for carbon accumulation than the biome average. Our results demonstrate that old‐growth forest loss remains the most important factor determining the carbon balance in the Brazilian Amazon. Understanding the implications of these findings will be essential for improving estimates of secondary forest carbon sequestration potential. More accurate quantification of secondary forest carbon stocks will support the production of appropriate management proposals that can efficiently harness the potential of secondary forests as a low‐cost, nature‐based tool for mitigating climate change. Secondary forests are increasing in the Brazilian Amazon and may be an important mechanism for reducing net carbon emissions. Using MapBiomas 3.1 and recently refined IPCC carbon sequestration estimates, we mapped the age and extent of secondary forests in the Brazilian Amazon. In 2017, these secondary forests stored 0.33 ± 0.05 billion Mg of above‐ground carbon but had offset just 9.37% of old‐growth emissions since 1985. Our results demonstrate that old‐growth forest loss remains the most important factor determining the carbon balance in the Brazilian Amazon.