Abstract The mixing state of black carbon (BC) strongly influences its lifetime and radiative forcing over the Tibetan Plateau (TP). Previous observations, mostly conducted during the non‐monsoon season, suggested that thickly coated BC dominates due to long‐range transport from South Asia. Here we report near‐surface BC mixing state measurements during the 2021–2023 summer monsoons at seven sites across the central–southern TP. We find that BC predominantly exists as bare/thinly‐coated state (externally mixed), with fractions of 73.2%–93.7%, much higher than earlier reports (20%–60%). This mainly arises from suppressed transport from South Asia during the monsoon season and intrinsically slow coating‐growth over the TP. The prevalence of externally mixed BC implies weaker absorption enhancement and CCN activity. These results underscore the need to represent seasonal variability of BC mixing state over the TP in climate models.