<p>The application of digital technology is a crucial means to achieve low-carbon development goals, with supplier firms’ digital transformation driving supply chain optimization as a key pathway to this goal. This study systematically examines the forward spillover effects of supplier firms’ digital transformation. Using panel data from A-share listed companies in China over the period 2007–2022, we focus on vertical supply chain linkages. The results indicate that supplier firms’ digital transformation significantly reduces customer firms’ environmental pollution emissions, a conclusion that remains robust across various tests. Moreover, we find that this effect is asymmetric: it is significantly stronger when suppliers’ digitalization levels exceed those of their customers. Heterogeneity analysis shows that the pollution-reduction effect is stronger when customer firms are located in regions with lower resource endowments, have higher supply-chain dependence, operate in non-heavy-pollution industries, or are non-state-owned enterprises. Mechanism analysis demonstrates that supplier digitalization achieves these gains by enhancing green technological innovation and increasing external stakeholder attention. The findings provide a foundation for fully leveraging the environmental potential of digital transformation within supply chains and offer important insights into advancing supply chain modernization and promoting green, high-quality development.</p>