Localized surface plasmon resonance (LSPR) in gold nanoparticles (AuNPs) provides an ultrasensitive probe of interfacial hydration dynamics, yet the quantitative role of ionic-strength-modulated hydration shells in governing both non-radiative damping and spectral shifts remains unresolved. A predictive framework is introduced that couples electrostatic theory with experimental LSPR measurements spanning seven orders of magnitude in buffer ionic strength I (10−9– 10−2 M). The hydration shell (HS) thickness is shown analytically and empirically to scale as I−1/2, with this single parameter accounting for over 90% of the variance in optical absorbance (r = 0.94). Concurrently, resonance-wavelength shifts are shown to arise from the combined effects of refractive index (RI) changes (r = 0.79) and ionic screening (r = 0.60). Principal component analysis (PCA) distills these five interrelated variables into two orthogonal design levers—HS thickness versus solution conditions—enabling independent tuning of LSPR signal amplitude and spectral position. This unified model elucidates the fundamental physics of ionic-strength-driven LSPR modulation and also furnishes a robust calibration protocol for nanoparticle-based sensors operating in complex media. Future extensions to diverse ion chemistries, pH environments, and nanoparticle geometries will not only tailor this framework for advanced plasmonic assays but also empower the rational design of nanoparticle-based solid-liquid interface devices and applications.