Purpose: A split-mouth randomized controlled trial (RCT) to evaluate the impact of a 2% proanthocyanidin (PAC) pretreatment liner on the clinical durability of posterior composite restorations. Materials and methods: A parallel double-blinded split-mouth RCT adhering to Consolidated Standards of Reporting Trials (CONSORT) guidelines was conducted. A 2% PAC solution was prepared for pretreatment. A total of 44 participants meeting inclusion criteria received composite restorations, each receiving one restoration with PAC liner and one without (test and control). Clinical evaluations include esthetic, functional, and biological aspects over a 6-month and 1-year follow-up period using Fédération Dentaire Internationale (FDI) criteria. Fisher exact test was employed to analyze the differences in ratings between the two groups. Results: No significant differences were observed between the two groups (p < 0.05). Staining was minimal and comparable between the two groups; neither group experienced postoperative sensitivity or recurrent caries. Both groups exhibited excellent outcomes in surface luster, color match, translucency, and marginal adaptation, with no significant differences between groups. Staining, postoperative sensitivity, and recurrent caries were minimal and comparable. General and oral health remained favorable in both groups during follow-up. Conclusion: Around 2% PAC pretreatment enhances the clinical durability of posterior composite restorations, leading to excellent esthetic, functional, and biological outcomes. PAC represents a promising dentin biomodification agent for improving restoration performance and longevity.