This article discusses the Macao boundary question, a significant diplomatic issue between China and Portugal from the 1860s to the 1920s. It analyses the various obstacles that prevented the delimitation of the Macao border requested by the Portuguese from taking place. Lack of documentary evidence to support the Portuguese’s occupation of Macao, the domestic opinion, and local commercial interests of Xiangshan County, Guangdong, constituted essential obstacles. Meantime, it discusses the link between China’s increasing awareness of the concept of sovereignty in the late nineteenth and early twentieth centuries and the boundary question, contemporary Chinese foreign relations with Portugal, Great Britain, and the United States, as well as the disagreement between the central and provincial governments in China regarding the boundary question. The article further argues that based on the occupation principle in international relations, the Portuguese in reality obtained what was negotiable, although a written agreement was not concluded when the nine meetings on the boundary issue were finished. The Chinese acquiescence led to the prolonged Portuguese occupation of Taipa and Coloane. The Macao boundary question reveals the delicate historical relationships between China and Portugal and illustrates the complex nature of international relations.