$Q\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}m$ $m\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}s$, an emerging field in artificial quantum systems, considers the interactions of electromagnetic waves (photons) with magnetic oscillations (magnons). Here the fundamentally weak coupling between quasiparticles challenges the development of on-chip devices. This study demonstrates the on-chip realization of ultrastrong photon-magnon coupling, using superconducting structures with reduced photon phase velocity. Attaining a coupling ratio relatively close to unity, it is found that the studied structure obeys the Hopfield interaction model, with plasmonic contribution to the energy of the magnon-polariton system.