My dissertation examines the central role that Taiwan played in Japan's southern expansion from 1895 to 1945. As Japan's first overseas colony, Taiwan was the maritime gate through which the Japanese extended their economic and geopolitical interests in South China and Southeast Asia. Under the administration of the Government-General of Taiwan, the Taiwanese port cities of Keelung and Kaohsiung emerged as the main Japanese entrepots for the circulation of people, goods, and ideas across the East and South China Seas. While scholarship on Japanese imperialism has predominantly concentrated on Korea and Northeast China, my research reorients our geographic focus to the understudied southern half of Japan's empire. Drawing on archival sources in Tokyo, Taipei, and Xiamen, as well as the methodologies of maritime and migration studies, I look at how Japanese imperialism shaped Taiwan's relations with its southern neighbors, and in turn how Taiwan impacted policies and conceptions of Japan as a maritime power. In exploring the role of colonial Taiwan as a base for "informal" (economic) and "formal" (military) imperialism, I focus on the perspectives of Japanese officials and colonists, Taiwanese subjects, and local and overseas Chinese. First, I illustrate how Japanese officials aimed to mobilize, limit, and monitor the movement of people--officials, entrepreneurs, laborers, soldiers--across the China Seas to extend Japan's economic interests in the south. Colonial rule not only transformed trade and migration networks between Japan and Taiwan, but also those between Taiwan, South China, and Southeast Asia, much of it at the expense of the Chinese and Western powers. Second, I examine Taiwan as a military and administrative base for surveying, and ultimately occupying, South China and Southeast Asia in the 1930s-1940s. Lastly, I analyze the sea- and island-based ideologies of southern expansion that shaped Japanese and Taiwanese conceptions of nation and empire.