After the Japanese colonization of Taiwan in 1895, colonial and diplomatic officials sought to encourage, regulate, and surveil the movement of individuals from Taiwan to the south China treaty-ports by conferring upon those who traveled there the legal designation Taiwan sekimin, or “registered Taiwanese.” Japanese officials and sekimin alike fashioned the Taiwan’s inhabitants, their capital, their socio-economic networks, and Taiwan’s colonial institutions as the basis for expanding the Japanese empire’s political and economic influence. This legal status afforded sekimin the extraterritorial protection of local Japanese consulates and subjected them to consular oversight. Over time, the category of Taiwan sekimin was expanded to include local and overseas Chinese whose support Japanese officials sought to garner. This dissertation charts the transformation of Taiwan sekimin as a juridical and social category and argues that it was central to Japanese colonial policy in Taiwan and imperial ambitions in south China. By tracing these changes, this dissertation shows how efforts by Japanese and Chinese officials, as well as by sekimin themselves, drew upon and reshaped the existing social and commercial networks that linked Taiwan to the south China treaty-ports and conditioned Japanese imperial and Chinese imperial and national state formation in south China. Taiwan sekimin ranged from wealthy elites, petty merchants, and doctors and other professionals trained in colonial Taiwan, to young anti-colonial activists drawn to China, criminal elements who formed gangs, and disreputable proprietors of opium and gambling establishments. Diverse though the category was, the status of Taiwan sekimin became, at times, the basis of individuals’ appealing for Japanese consular protection, and at others, the basis of Japanese officials’ laying claim to exercising jurisdiction over individuals considered Japanese subjects. By exploring how Taiwan sekimin individuals both supported and challenged the ideologies and institutions of the Japanese empire at its margins, this dissertation reveals their role in entrenching a Japanese imperial sphere across and beyond the region between 1895 and the start of the Second Sino-Japanese War in 1937. The legal and spatial bounds of Taiwan sekimin as a juridical and social category were central to intra-imperial and inter-imperial contestations for power in south China. Contention over the Japanese empire’s economic and political ambitions led to contestation over the legal boundaries of Taiwan sekimin between Japanese colonial officials in Taiwan and local consular officials, who sought to regulate the mobility of people, ideas, and capital between Taiwan and the treaty-ports. Over time, Japanese officials also sought to channel the support of sekimin through new institutions. These institutions expanded the spatial scope of jurisdictional contests within and beyond the treaty-ports and thus the scope of imperial power; these institutions also rendered Japanese imperial ambitions more contingent on the support of the sekimin. Chinese local, national, and diplomatic officials also actively challenged the legality of sekimin status and the inclusion of individuals these officials considered Chinese nationals under its purview, particularly after the rise to power of the Chinese Nationalist Party (Guomindang) in the late 1920s. In the 1930s, the concept of Taiwan sekimin was increasingly at odds with Chinese national conceptions of social and economic order. This dissertation shows that, in this context, conflicts involving sekimin were not just local scuffles to be resolved on the interpersonal level but laden with ideological import, leaving the sekimin caught between the logics of empire and nation. This dissertation draws on Japanese- and Chinese-language materials from Japan, Taiwan, and China. It reads official sources “along the grain” to reveal the logic that organized knowledge production about the sekimin and “against the grain” to reconstruct a history largely beyond the purview of bureaucratic institutions. By exploring the competing inter- and intra-imperial claims to authority over Taiwan sekimin, this dissertation argues that jurisdictional contestation had legal and spatial implications in linking Chinese national and Japanese imperial state formation in south China.