<p>This in vitro study aimed to evaluate the preventive effects of whitening mouthrinses against coffee-induced discoloration and to assess their influence on the optical properties—color difference (Δ<i>E</i>₀₀), translucency parameter (ΔTP), and whiteness index difference (ΔWID)—of different resin composite materials. Three resin composites were tested: Filtek One Bulk Fill (FOB), Charisma Diamond One (CDO), and Estellite Sigma Quick (ESQ). Specimens were immersed daily in coffee, followed by exposure to three whitening mouthrinses—Listerine Advanced White (LAW), Colgate Optic White (COW), and R.O.C.S. Whitening Mouthrinse (ROC)—for 3&#xa0;months. Distilled water served as the negative control. Color measurements were conducted with a spectrophotometer on standardized black, white, and gray backgrounds. Δ<i>E</i>₀₀ values were analyzed using mixed-design ANOVA or Kruskal–Wallis tests depending on data distribution, while ΔTP and ΔWID values were evaluated using appropriate non-parametric tests (<i>p</i> &lt; 0.05). All whitening mouthrinses significantly reduced coffee-induced discoloration (<i>p</i> &lt; 0.05). LAW and COW, both containing hydrogen peroxide, exhibited the lowest Δ<i>E</i>₀₀ values (≈ 2.0–2.3), while ROC showed limited efficacy. Whitening mouthrinses did not significantly affect translucency (ΔTP) values (<i>p</i> &gt; 0.05), whereas coffee exposure resulted in a statistically significant reduction in translucency. Whitening rinses produced a mild increase in whiteness (ΔWID ≈ + 1.0– + 1.6), below the clinical perceptibility threshold (ΔWID ≥  + 2.0). Among materials, the nanohybrid composite ESQ demonstrated the most stable optical behavior across all parameters. Hydrogen peroxide–containing whitening mouthrinses exerted a preventive effect against coffee-induced discoloration without producing clinically perceptible whitening, thereby helping to maintain the optical stability of composite restorations.</p>