Drainage threatens the vast soil organic carbon (SOC) reservoirs in wetlands, but uncertainties in SOC responses across different wetlands hinder an accurate estimate of their carbon dynamics and climate mitigation capacity. Here we conducted a transcontinental pairwise survey, including 29 pairs of drained and waterlogged sites across low- to high-latitude wetlands in China and Finland after decades of drainage. A total of 2437 soil samples from 188 profiles were analyzed using a high-resolution equivalent ash mass method. We showed that drainage reduced SOC stocks by 32% ± 10% in carbon-rich wetlands due to Sphagnum decay and enhanced decomposition, while the fixed-depth comparison underestimated the SOC stocks by up to 12 folds. Conversely, SOC stocks increased by 46% ± 27% in carbon-poor wetlands due to enhanced plant inputs. Upscaling analysis suggests that, over the past century, drainage alone (without post-drainage disturbances) has led to a 1.80 Gt SOC gain in China, exceeding afforestation-driven SOC gains in northern China during the past three decades by 7.5 folds. Our findings reveal the divergent transcontinental wetland SOC responses to drainage, highlighting the overlooked climate mitigation potential of carbon-poor wetlands under global changes.