C-type asteroids are the presumed home to carbonaceous chondrites, some of which contain abundant life-forming volatiles and organics. For the first time, samples from a C-type asteroid (162173 Ryugu) were successfully returned to Earth by JAXA's Hayabusa2 mission. These pristine samples, uncontaminated by the terrestrial environment, allow a direct comparison with carbonaceous chondrites. This study reports the stable K isotopic compositions (expressed as δ⁴¹K) of Ryugu samples and seven carbonaceous chondrites to constrain the origin of K isotopic variations in the early Solar System. Three aliquots of Ryugu particles collected at two touchdown sites have identical δ⁴¹K values, averaged at −0.194 ± 0.038‰ (2SD). The K isotopic composition of Ryugu falls within the range of δ⁴¹K values measured on representative CI chondrites, and together, they define an average δ⁴¹K value of −0.185 ± 0.078‰ (2SE), which provides the current best estimate of the K isotopic composition of the bulk Solar System. Samples of CI chondrites with δ⁴¹K values that deviate from this range likely reflect terrestrial contaminations or compositional heterogeneities at sampled sizes. In addition to CI chondrites, substantial K isotopic variability is observed in other carbonaceous chondrites and within individual chondritic groups, with δ⁴¹K values inversely correlated with K abundances in many cases. These observations indicate widespread fluid activity occurred in chondrite parent bodies, which significantly altered the original K abundances and isotopic compositions of chondrules and matrices established at their accretion.