Ultrastrong exciton-photon coupling of Frenkel molecular excitons is demonstrated at room temperature in a metal-clad microcavity containing a thin film of 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl)fluorene. A giant Rabi splitting of Ω ∼ 1 eV is measured using angle-resolved reflectivity and bright photoluminescence is observed from the lower polariton branch. To obtain the virtual photon and exciton content of the polariton ground state, the results are interpreted in terms of the full Hopfield Hamiltonian, including anti-resonant terms. Also included is an analytical treatment of the often ignored and sometimes misinterpreted TM-polarized metal-insulator-metal plasmon-polariton. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.