High-latitude and high-altitude regions contain vast stores of permafrost carbon. Climate warming may result in the release of CO<sub>2</sub> from both the thawing of permafrost and accelerated autotrophic respiration, but it may also increase the fixation of CO<sub>2</sub> by plants, which could relieve or even offset the CO<sub>2</sub> losses. The Tibetan Plateau contains the largest area of alpine permafrost on Earth. However, the current status of the net CO<sub>2</sub> balance and feedbacks to warming remain unclear, given that the region has recently experienced an atmospheric warming rate of over 0.3 °C decade<sup>-1</sup> We examined 32 eddy covariance sites and found an unexpected net CO<sub>2</sub> sink during 2002 to 2020 (26 of the sites yielded a net CO<sub>2</sub> sink) that was four times the amount previously estimated. The CO<sub>2</sub> sink peaked at an altitude of roughly 4,000 m, with the sink at lower and higher altitudes limited by a low carbon use efficiency and a cold, dry climate, respectively. The fixation of CO<sub>2</sub> in summer is more dependent on temperature than the loss of CO<sub>2</sub> than it is in the winter months, especially at higher altitudes. Consistently, 16 manipulative experiments and 18 model simulations showed that the fixation of CO<sub>2</sub> by plants will outpace the loss of CO<sub>2</sub> under a wetting-warming climate until the 2090s (178 to 318 Tg C y<sup>-1</sup>). We therefore suggest that there is a plant-dominated negative feedback to climate warming on the Tibetan Plateau.