Firms are inevitably involved in both product and financial markets, and their strategic decisions in one domain unintentionally influence outcomes in the other. This study examines the spillover effect of firms' product market behavior, such as advertising, on financial market outcomes, focusing on its role in shaping the relationship between share repurchase and stock mispricing. Using the dataset of US-listed firms from 1983 to 2016, we find that higher advertising expenses significantly amplify the relationship between share repurchase and stock mispricing. This effect is more pronounced in firms with high investors' attention and those in consumer-product industries. Our mechanism analyses reveal that advertising influences the relationship by shifting investors' attention (making investors ignore financial information, weakening their ability to detect and respond to mispricing signals) and intensifying investors' opinion divergence (heightening the risk of capturing mispricing due to less pricing consensus, delaying investors' responses to mispricing signals), and thereby allow firms to capitalize on temporary mispricing via repurchase. Robust tests, including an instrumental variable approach and stricter fixed-effect models, confirm the causal nature of these findings. We also find that our results are not driven by the repurchase incentives during crisis or recession periods. Our results highlight a novel link between corporate advertising strategies and financial market behavior, providing new insights into the interaction between financial and non-financial market dynamics.