  Two-dimensional transition metal dichalcogenides exhibit strong optical
transitions with significant potential for optoelectronic devices. In
particular they are suited for cavity quantum electrodynamics in which strong
coupling leads to polariton formation as a root to realisation of inversionless
lasing, polariton condensationand superfluidity. Demonstrations of such
strongly correlated phenomena to date have often relied on cryogenic
temperatures, high excitation densities and were frequently impaired by strong
material disorder. At room-temperature, experiments approaching the strong
coupling regime with transition metal dichalcogenides have been reported, but
well resolved exciton-polaritons have yet to be achieved. Here we report a
study of monolayer WS$_2$ coupled to an open Fabry-Perot cavity at
room-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 data
are well described by a transfer matrix model appropriate for the large
linewidth regime. This work provides a platform towards observing strongly
correlated polariton phenomena in compact photonic devices for ambient
temperature applications.
