The filled skutterudite PrRu₄P₁₂ shows a metal–insulator (MI) transition at 62.3 K which is yet to be understood. It is believed that the Pr 4f electrons play an integral role in the MI transition however the order parameter associated with the Pr 4f shells remains elusive. In this study, we investigate PrRu₄P₁₂ using temperature dependent resonant soft x-ray diffraction in combination with x-ray absorption spectroscopy at the Pr M_4,5 edges. A resonant enhancement of the (100) reflection at the Pr M_4,5 edges signaling the order parameter of the Pr 4f shells was observed below T_MI, with a steady increase with decreasing temperature. The experimental spectra and subsequent analysis rule out the existence of magnetic, orbital and significant charge order, as well as any Pr lattice displacement. The ordered state below T_MI is likely the periodic electronic energy-level modulation of Pr 4f shells. Our results indicate that a synergistic coupling of Pr 4f states and lattice distortion plays an important role in the metal–insulator transition associated with charge density wave state formation.