Superradiant　phase　transitions　(SPTs)　are　important　for　understanding　light-matter　interactions　at　the　quantum　level,　and　play　a　central　role　in　criticality-enhanced　quantum　sensing.　So　far,　SPTs　have　been　observed　in　driven-dissipative　systems,　but　the　emergent　light　fields　did　not　show　any　nonclassical　characteristic　due　to　the　presence　of　strong　dissipation.　Here　we　report　an　experimental　demonstration　of　the　SPT　featuring　the　emergence　of　a　highly　nonclassical　photonic　field,　realized　with　a　resonator　coupled　to　a　superconducting　qubit,　implementing　the　quantum　Rabi　model.　We　fully　characterize　the　light-matter　state　by　Wigner　matrix　tomography.　The　measured　matrix　elements　exhibit　quantum　interference　intrinsic　of　a　photonic　mesoscopic　superposition,　and　reveal　light-matter　entanglement.