High-pressure environments provide a unique platform for tuning quantum phenomena, yet their applications in plasmonics remain underexplored. Here, we investigate the pressure-induced evolution of plasmons in black phosphorus films using infrared spectroscopy. Continuous pressure tuning of anisotropic plasmon resonances reveals the existence of in-plane hyperbolic plasmons, whose regime blueshifts into the mid-infrared range in the low-pressure A17 phase and collapses in the high-pressure A7 phase due to the suppressed anisotropy. Notably, we observed an exceptionally broad spectral range tuning of plasmon resonance frequency, ranging from $214{\mathrm{cm}}^{-1}$ to a maximum of $4751{\mathrm{cm}}^{-1}$, along with significant modulation in absorption intensity and anisotropy. Pronounced plasmonic anomalies at the transition point reveal phase coexistence, while the abrupt low-pressure plasmon onset signals a Lifshitz transition. Our findings establish high pressure as a novel method for tailoring plasmonic properties and unlocking new possibilities for reconfigurable nanophotonic devices.

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• Plasmonics
• Pressure effects
• Thin films
• Infrared spectroscopy