Using Chinese A-share listed firms from 2007 to 2024, this study examines whether supply chain shareholding (SCS) affects stock price synchronicity. We find that SCS significantly reduces stock price synchronicity. This finding remains robust to a series of robustness and endogeneity tests. Mechanism tests provide evidence consistent with information asymmetry and agency costs as two channels linking SCS to lower stock price synchronicity. Heterogeneity analyses show that the effect is more pronounced among firms with low supply chain concentration and those operating in unfavorable business environments. Overall, these findings identify SCS as a relationship-based governance arrangement that improves capital market information efficiency, particularly where relational governance and informal institutional substitution are more valuable.