Two-dimensional atomic crystals of graphene, as well as transition-metal dichalcogenides, have emerged as a class of materials that demonstrate strong interaction with light. This interaction can be further controlled by embedding such materials into optical microcavities. When the interaction rate is engineered to be faster than dissipation from the light and matter entities, one reaches the ‘strong coupling’ regime. This results in the formation of half-light, half-matter bosonic quasiparticles called microcavity polaritons. Here, we report evidence of strong light–matter coupling and the formation of microcavity polaritons in a two-dimensional atomic crystal of molybdenum disulphide (MoS 2 ) embedded inside a dielectric microcavity at room temperature. A Rabi splitting of 46 ± 3 meV is observed in angle-resolved reflectivity and photoluminescence spectra due to coupling between the two-dimensional excitons and the cavity photons. Realizing strong coupling at room temperature in two-dimensional materials that offer a disorder-free potential landscape provides an attractive route for the development of practical polaritonic devices.