We examine whether a supplier’s earnings convey information relevant for valuing other suppliers that share a major customer, thereby generating horizontal information transfers. Sorting firms sharing a major customer by the timing of their earnings announcements (EAs), we find a significantly positive relation between the first announcer’s earnings news and other suppliers’ abnormal returns when the customer’s EA has not yet been released. We rule out the possibility that this finding is driven by intra-industry information transfers or production complementarities, rather than by the shared customer connection. The magnitude of these externalities is larger when the customer’s information environment is poorer, the customer’s financial health is weaker, the customer-supplier relational dependence is greater, and the economic ties between firms in the supply chain are stronger. Overall, our results suggest that investors exploit horizontal information spillovers to value firms that share a customer when information externalities from customer disclosures are limited.