Compared to gapless graphene, the extraordinary electronic structure of single‐layer MoS2 may lead to many potential applications such as field‐effect transistors, optoelectronic and spin‐valley devices. All these applications are closely related to the thermal properties. In semiconductors, heat is carried by the atomic vibrations quantized as phonons. To capture the specifics of the phonon transport in single‐layer MoS2, we used the phonon‐scattering diagrams, and treated the three‐phonon interaction processes directly while considering all phonon relaxation channels allowed by the energy and momentum conservation in two‐dimensional Brillouin zone. The intrinsic thermal conductivity of MoS2 is obtained at 101 W/mK. We found that the three‐phonon processes with much shorter relaxation time than the isotopic scattering dominate the total relaxation time, and the presence of naturally‐occurring isotopes leads to a 10% reduction of the total lattice thermal conductivity. With a high thermal conductivity, MoS2 looks promising as an emerging electronic device material.