Exciton-polaritons are perspective platform for realizing ultrafast and strong optical modulations, which are necessary for the plant of applications. However, exciton-polaritons are studied mostly for semiconductor quantum wells inside vertical Bragg cavities, which limits it to the cryogenic temperatures and prevents planar realizations, which can be a problem for real-world applications. Recently, perovskites become one of the perspective materials for room-temperature strong light-matter coupling regime due to their unique physical properties. In this work, we experimentally demonstrate for the first-time room-temperature exciton-polaritons in planar halide perovskite photonic crystal slab fabricated by a nanoimprint lithography method. We experimentally measured polariton dispersion from angle-resolved photoluminescence spectra and confirm the strong light-matter coupling regime at room temperature and lower. Also, we studied the temperature dependence of the exciton energy level in MAPbBr3 and the light-matter coupling coefficient. The obtained dependences can be attributed to polaron effects in this material. The results can become the basis of further research on perovskite exciton-polaritons in planar photonic cavities.