Two-dimensional transition metal dichalcogenides exhibit strong opticaltransitions with significant potential for optoelectronic devices. Inparticular they are suited for cavity quantum electrodynamics in which strongcoupling leads to polariton formation as a root to realisation of inversionlesslasing, polariton condensationand superfluidity. Demonstrations of suchstrongly correlated phenomena to date have often relied on cryogenictemperatures, high excitation densities and were frequently impaired by strongmaterial disorder. At room-temperature, experiments approaching the strongcoupling regime with transition metal dichalcogenides have been reported, butwell resolved exciton-polaritons have yet to be achieved. Here we report astudy of monolayer WS$_2$ coupled to an open Fabry-Perot cavity atroom-temperature, in which polariton eigenstates are unambiguously displayed.In-situ tunability of the cavity length results in a maximal Rabi splitting of$\hbar \Omega_{\rm{Rabi}} = 70$ meV, exceeding the exciton linewidth. Our dataare well described by a transfer matrix model appropriate for the largelinewidth regime. This work provides a platform towards observing stronglycorrelated polariton phenomena in compact photonic devices for ambienttemperature applications.
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