We investigate plasmon-plasmon (PP) coupling in the strongly interacting regimes by using a tunable plasmonic platform consisting of triangular Ag nanoprisms placed nanometers away from Ag thin films. The nanoprisms are colloidally synthesized using a seed-mediated growth method and having size-tunable localized surface plasmon polariton (SPP) resonances immobilized on Si 3 N 4 films. The PP coupling between the localized SPPs of metal nanoprisms and the propagating SPPs of the metal film is controlled by the nanoprism concentration and the plasmon damping in the metal film. Results reveal that Rabi splitting energy determining the strength of the coupling can reach up to several hundreds meV, thus demonstrating the ultrastrong coupling occurring between localized and propagating SPPs. The metal nanoparticle-metal thin film hybrid system over the square-centimeter areas presented here provides a unique configuration to study PP coupling all the way from the weak to ultrastrong coupling regimes in a broad range of wavelengths. Studying light-matter interaction at the nanoscale dimension with metallic nanostructures and metallic thin films has become an intense area of research due to the ability of metallic nanostructures and metallic ultrathin films to confine and concentrate light at the subwavelength scale by excitation of surface plasmon polaritons (SPPs) [1,2]. SPP-enabled enhancement of light-matter interaction has been further driven by the developments of improved top-down and bottom-up nanofabrication techniques and also by the ability to chemically synthesize quantum dots with size-and shape-tunable optical properties.To further widen the scope of the fundamental and applied science application of metallic plasmonic structures and boost the performance of the plasmonic devices, new hybrid platforms with easily tunable optical properties are required for engineering light-matter interaction at the nanoscale. This goal has been sought either by metal nanoparticle-metal film [3][4][5][6][7][8][9][10][11] or metal nanoparticle-metal nanoparticle [12-15] configurations. Previously, metal nanoparticles placed nanometers away from continuous metal thin films resulting in a number of enhanced optical effects and tunable plasmon resonances have been proposed and experimentally demonstrated as plasmonic platforms to study coupling of localized SPPs (LSPP) and propagating SPPs (PSPPs) [3][4][5][6][7][8][9][10][11]. Until now, the plasmonplasmon (PP) coupling observed in this configuration has been mostly studied using isotropic Ag nanoparticles having plasmon resonance at ∼400 nm or isotropic gold nanoparticles having plasmon resonance at ∼530 nm. In other cases, metal nanodisks and gratings have been used to study the PP coupling [9,10]. An optimum dielectric spacer layer thickness of around 10-30 nm was found to maximize PP coupling efficiency [3]. In the previous studies: (1) PP coupling has been investigated in the strong coupling regime, nevertheless, the extent of the coupling has not been reported; (2) most...