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Achieving in situ control of the coupling strength is essential for investigating how such hybridization evolves with the coupling strength. In this work, we use a symmetry-changing structural phase transition in lead halide perovskites to reversibly tune the phonon\u2013photon coupling strength, leveraging the fact that their phonon frequencies and oscillator strengths are dictated by lattice symmetry. Terahertz time-domain spectroscopy of MAPbI3 embedded in nanoslot cavities reveals three polariton branches above the critical temperature Tc \u2243 162.5 K and the emergence of an additional branch below Tc, activated by a new phonon mode in the low-temperature phase. The full dispersion is accurately reproduced using a multimode Hopfield model, confirming that all normalized coupling strengths remain in the ultrastrong coupling regime. These results demonstrate symmetry-controlled tuning of ultrastrong light\u2013matter coupling in optical cavities via temperature.<\/jats:p>","DOI":"10.1063\/5.0313120","type":"journal-article","created":{"date-parts":[[2026,3,10]],"date-time":"2026-03-10T10:48:52Z","timestamp":1773139732000},"update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":2,"title":["Symmetry-controlled ultrastrong phonon\u2013photon coupling in a terahertz cavity"],"prefix":"10.1063","volume":"164","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-2331-4526","authenticated-orcid":false,"given":"Dasom","family":"Kim","sequence":"first","affiliation":[{"name":"Department of Electrical and Computer Engineering, Rice University 1 , Houston, Texas 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