Variation in aboveground live tree biomass carbon (AGC) stocks is poorly understood in tropical montane forests1, especially in African nations where montane forests often represent most of the extant evergreen old-growth forest cover. Although data are few, since primary productivity is temperature-mediated and cloud immersion, wind and steep slopes constrain tree height2, AGC is widely assumed to be lower in tropical montane than lowland forests. To test this, we assembled and analysed a new dataset of structurally intact old-growth forests (“AfriMont”) spanning 44 montane sites in 12 African countries, and compared findings with old-growth lowland forests in the African Tropical Rainforest Observation Network (AfriTRON). We find that montane sites in the AfriMont plot network have a mean AGC-stock of 149.4 Mg C ha-1 (95% CI 137.1-164.2), comparable to lowland forests in the AfriTRON plot network and higher than averages from plot networks in montane and lowland forests in the Neotropics. Notably, our results are substantially higher than the IPCC default values for these forests in Africa (89.3 Mg C ha-1)3. The distinctive structure of African lowland forests (low stem density and high abundance of large trees4-6) is mirrored in montane forests. This important carbon store is endangered, we find that 0.8 million ha of old-growth African montane forest have been lost since 2000. Our findings highlight the urgent need for conserving these biodiverse7,8 and carbon-rich ecosystems. We provide country-specific montane forest AGC estimates modelled from our plot network to help guide forest conservation and reforestation interventions.