In-stent restenosis (ISR) develops primarily due to neointimal hyperplasia. Gallic acid (GA) has anti-inflammatory, antioxidant, and cardioprotective effects. This study sought to investigate the effects of GA on neointimal hyperplasia and proliferation and migration of vascular smooth muscle cells (VSMCs) in a pig ISR model. In vitro proliferation and migration experiments were confirmed, after VSMCs were treated with platelet- derived growth factor (PDGF-BB) and GA (100 mM) using a 3?(4,5?dimethylthiazol)? 2,5?diphenyltetrazolium bromide (MTT) assay and a scratch wound assay for 24 hours and 48 hours. A bare metal stent (BMS) was implanted in the pig coronary artery to induce ISR with overdilation (1.1-1.2:1), and GA (10 mg/kg/day) was administered for 4 weeks. At the 4-week follow-up, optical coherence tomography (OCT) and histopathological analyses were performed. GA decreased the proliferation of VSMCs by PDGF-BB for 24 hours (89.24±24.56% vs. 170.04±19.98%, p＜0.001) and 48 hours (124.87±7.35% vs. 187.64±4.83%, p＜0.001). GA inhibited the migration of VSMCs induced by PDGF-BB for 24 hours (26.73±2.38% vs. 65.38±9.73%, p＜0.001) and 48 hours (32.96±3.04% vs. 77.04±10.07%, p＜0.001). Using OCT, % neointimal hyperplasia was shown to have significantly decreased in the GA group com...