This dissertation examines the life and monastic career of Shaku Sōen (1860-1919) in Japan, Sri Lanka, and the United States from the mid-nineteenth through the early twentieth centuries. It draws on a rich documentary record in Japanese, literary Chinese (kanbun), and English to analyze the social, doctrinal, and ritual life of the Rinzai Zen monastic community. It traces the survival and revival of Rinzai Zen to adapt to Japan’s nation and empire building through the Meiji period (1868-1912), and the negotiation with Zen’s pre-Meiji scriptures and practice and Western studies of Buddhism until the Taishō period (1912-1926). Previous studies tend to approach the modernization of Zen from a Westernization perspective or that of Japanese nationalism and colonial expansion. To bring the two perspectives in conversation with each other and add new insights, this study reconstructs Sōen’s life and career in association with his dharma lineage and three major Zen monasteries—Myōshinji, Engakuji, and Kenchōji—and their capillary network of branch temples throughout Japan. It also traces Sōen’s three-year study in Sri Lanka (Ceylon) and his major international travels to Thailand (Siam), China, Korea, the United States, among other places, that shaped his conception of Zen in the modern world. It sheds new light on the process of preparation that he and his colleagues had made to participate in the World’s Parliament of Religions in Chicago in 1893. This historical and transnational study of Sōen shows that he and his peers strove to modernize in the Western paradigm and identify Zen with nationalistic interests of the state, while preserving and reviving pre-Meiji scriptures and practices. They presented Zen from a Japanese cultural perspective and make Zen compatible with Japanese nationalism and militarism. Their collaboration with Ceylonese Theravādins and Westerners interested in Buddhism and Japanese culture encouraged the spread of Japanese Mahāyāna Buddhism to the West while supporting Zen revival in Japan. Sōen and his students including D. T. Suzuki (1870-1966) introduced Zen teachings to the United States, setting the terms for Zen’s reception in the West through the twentieth century. It suggests that the modernization of Zen was not simply embracing the Western influences but was also a creative process for Zen monks to harness both the domestic and international resources to position themselves in the modern world.