Grain boundaries (GBs) are inevitable defects in large-area MoS₂ samples but they play a key role in their properties, however, the influence of grain misorientation on thermal transport has largely remained unknown. Here, the critical role of misorientation angle in thermal transport characteristics across 5|7 polar dislocation-dominated GBs in monolayer MoS₂ is explored using nonequilibrium molecular dynamics simulations. Results show that thermal transport characteristics of defective GBs are greatly dictated by the misorientation angle, with “U”-shaped thermal conductance as misorientation angle varying from around 5.06–52.26°, as well as by GB energy, 5|7 dislocation type and the grain size. Such unique thermal transport across GBs is primarily attributed to rising phonon-boundary softening and scattering with increasing dislocation density at GBs or GB energy, as well as an increase in localized phonon modes. The study establishes the fundamental relationship between GB and the thermal properties of single-layer MoS₂ and highlights the vital role of GBs in designing efficient thermoelectric and thermal management transition metal dichalcogenides.