This dissertation examines Chinese Buddhism in maritime Southeast Asia; it also considers the history of Chinese migration and transregional religious circulations in the twentieth century. I use the religious careers of three Chinese monks—Chuk Mor (Zhumo 竺摩, 1913-2002), Yen Pei (Yanpei 演培, 1917-1996), and Ashin Jinarakkhita (Tizheng 體正, 1923-2002)—as case studies to explore the movements, exchanges, and innovation of Buddhist knowledge and institutions in the Malay Archipelago. In doing so, this dissertation has two primary goals. The first is to bring Chinese Buddhism into the study of Southeast Asia and demonstrate that Chinese diasporic monks were significant agents in disseminating Buddhist ideas in maritime Southeast Asia. I highlight the transnational circulations of people, ideas, and resources between Greater China and Southeast Asia. The second goal of this dissertation is to contribute to the literature that critiques the “colonial/western transformation” model in the study of Buddhism and modernity in Asian societies, and reveal that overseas Chinese monks were important actors in making maritime Southeast Asia a site of Buddhist modernism. This study seeks to situate these Buddhist monks and their transnational networks within a broader context of Chinese migration to Southeast Asia, the Buddhist reform movement in Republican China (1912-1949), the Second World War, the emergence of Communist China in 1949, and decolonization and nation-building in Indonesia, Malaysia, and Singapore during the second half of the twentieth century. This study argues for the need to broaden the category of “Southeast Asian Buddhism” beyond Theravāda Buddhism on mainland Southeast Asia to include South China Sea Buddhism in the maritime region of Southeast Asia. By South China Sea Buddhism, I refer to the varied forms of Buddhism in maritime Southeast Asia that use Mandarin Chinese, Southern Chinese dialects, and Southeast Asian languages in their liturgy and scriptures. Focusing on the histories of the relationships between migratory circulations and Buddhist modernism, this study seeks to contribute to the literature on Southeast Asian and Chinese Buddhism, Southeast Asian history, Chinese history, Buddhist modernism, and Chinese diasporic networks.