<p>We propose and analyze a novel plasmonic multi-resonator perfect absorber based entirely on an all-metal Cu grating structure for high-performance optical sensing applications. The design features a continuous Cu substrate with two identical grating exhibiting seven distinct narrowband resonances spanning the near-infrared region (1270–1990 nm) with absorption efficiencies exceeding 90%. With an ultra-narrow linewidth (FWHM = 0.0188 nm) and an outstanding Q-factor (≈ 10⁵), the highest-order resonance (P7) exhibits a perfect absorption value at 1991.312 nm, guaranteeing remarkable spectrum selectivity and sensing resolution. To enable tailored sensing capabilities, systematic studies reveal that adjusting geometric features such as the grating height and the spacing between gratings can precisely tune the resonance wavelength while maintaining strong absorption and narrow linewidths. Sensitivity analysis against refractive index variations in the surrounding medium indicates a high sensitivity (<i>S</i> ≈ 1991.311 nm/RIU), an outstanding figure of merit (FOM ≈ 1.06 × 10⁵), and a low detection limit on the order of 10⁻⁶ RIU. The absorber’s strong sensitivity to small changes in refractive index, including those caused by gas analytes such as air, helium, nitrogen, and carbon dioxide, highlights its promising potential for use in multiplexed and selective biochemical and gas sensing applications. The use of an all-metal configuration supporting multiple high-Q resonances is unique among current absorber designs. This structure combines simplicity, tunability, and multi-wavelength operation in a single material platform, offering a novel approach to achieving high-performance sensing without relying on hybrid or multilayered materials. This work advances the development of compact, tunable, and ultra-sensitive plasmonic sensors leveraging high-Q narrowband resonances in all-metal architectures.</p>