Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr3 by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass of 0.26 +/- 0.02 m(e), 50% heavier than the bare mass m(0) = 0.17 m(e) predicted by density functional theory. Calculations of the electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Frohlich coupling parameter alpha = 1.81. A good agreement with our experimental data is obtained within the Feynman polaron model, validating a viable theoretical method to predict the carrier effective mass of LHPs ab initio.