Abstract: Micro/nanoplastics (M/NP) are widely distributed in aquatic environments, are significant threats to aquatic life as carriers of pollutants. Ultraviolet–visible (UV–Vis) spectrophotometry, due to simplicity and sensitivity, holds potential for environmental analysis. The UV–Vis absorption spectra (190–1100 nm) of polystyrene nanoplastics (PSNP, φ = 100 nm) were measured. Spectral interference from NaCl, MgCl₂ and CaCl₂ were analyzed. PSNP and these inorganic salts showed overlap in absorption in the region of 190–340 nm. Increases in Cl⁻ concentration were found to induce redshifts in resultant spectra, leading to deviations from the Lambert–Beer law, to interfer with the quantitative determination of PSNP. Redshifts were corrected to construct a nonlinear quantitative model, with goodness-of-fit (R_adj.²) for NaCl, MgCl₂, and CaCl₂ solutions improved to 0.998 - 0.999. Model validation using mixed systems indicated that predicted absorbance of PSNP in NaCl solution reached 98.2% ± 3.0% of standard values. However, when MgCl₂ and CaCl₂ concentrations approached or exceeded critical coagulation concentration (CCC), aggregation and sedimentation of PSNP occurred, resulting in a significant decrease in absorbance. Overall, the proposed redshift-corrected nonlinear model could effectively enhance accuracy of quantitative M/NP analysis of environmental waters.