The interaction of light with correlated quantum matter in two-dimensional (2D) materials offers a powerful platform for exploring emergent quantum phenomena. Here, we report the realization of Landau polaritons in a monolayer tungsten diselenide (${\mathrm{WSe}}_2$) integrated into an optical microcavity. By embedding a two-dimensional electron gas (2DEG) in the monolayer and applying a perpendicular magnetic field, we achieve strong coupling between cavity photons and interband Landau level (LL) transitions, giving rise to hybrid light-matter quasiparticles. These Landau polaritons exhibit valley-dependent oscillations in the coupling strengths and resonance energies, driven by the electrical and magnetic filling of valley-contrasting Landau levels. Moreover, intervalley correlations between LL transitions and opposite-valley electrons lead to nonlinear renormalization of both LL transition energies and Landau-polariton coupling strengths. Our results establish a new paradigm for cavity quantum electrodynamics in correlated 2D systems and demonstrate control of Landau polaritons via the valley degree of freedom, opening new avenues for light-mediated manipulation of quantum phases.

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• Doping effects
• Landau levels
• Polaritons
• Valley degrees of freedom
• Transition metal dichalcogenides
• Two-dimensional electron system
• Reflectivity