After the last Japanese embassy sailed to Tang dynasty China in 838, no official diplomats from Japan visited China until 1403, when the Ashikaga bakufu dispatched a delegation to the Ming dynasty to resume formal diplomatic relations. During this half millennium, monks and merchants from both China and Japan not only sustained but also furthered their communication across East Asian waters. Buddhist trade—the commercial exchange of objects for Buddhist uses, with monks and monasteries as participants—occupied a prominent position in Sino-Japanese trade between 838 and 1403. Previous scholarship has shown that the flows of commodities and personnel between China and Japan continued even after Japan had stopped paying tribute to China (Mori 1975; von Vershuer 2006; Enomoto 2007). However, why did the monks and monasteries participate in the trade? What advantages did Buddhist figures and institutions enjoy in the long-distance trade? To answer these questions, this study will focus on Buddhist traders, including monks, merchants (who were also lay Buddhists), and monasteries. During this diplomatically closed era, Buddhist traders used their religious resources as an asset to bring in trade partners and cut down transaction costs. Furthermore, these religious networks supported trade networks and facilitated commercial and cultural exchanges between the continent and the archipelago. In contrast with previous scholarship, this study pays close attention to the mechanisms of trade to show how religious networks and commercial networks gradually became integrated. Trust, credit, and contract are all important factors in long-distance trade and Buddhist traders transformed the trust embedded in religious networks to credit to facilitate transactions. Moreover, both textual evidence—The Record of Constructing the Tenryūji Monastery (1342)—and archaeological evidence—such as the Sinan shipwreck in South Korean waters—show that Japanese monasteries cooperated with merchants to trade with China and that they made agreements on how to divide their profits after the voyage. By integrating textual materials, transmitted images and objects, and archaeological discoveries, this project uses “religio-commercial networks” as a framework to reexamine the relationships between East Asian countries. Meanwhile, with a focus on the relationship between commerce and Buddhism, this project will provide a new dimension for investigation of trade between coreligionists and cross-cultural trade. It will also join a broader conversation with European historians who work on Jewish and Muslim traders.