In the United States during the long nineteenth century, the geographical societies of New York (American Geographical Society), Washington (National Geographic Society), and other cities were instrumental in the production, exchange, and popularisation of geographical knowledge about their country and the world. This chapter analyses how their lectures and journal articles encouraged and legitimised U.S. expansion into the Pacific region from 1851 to 1914. Their contributions covered the U.S. acquisition of the Philippines, Hawaii, and other Pacific islands after the Spanish-American War of 1898, and the construction of the Panama Canal under U.S. direction between 1904 and 1914. Using spatial arguments, the societies’ authors and lecturers pointed out the Pacific’s strategic importance for American trade with Asia and its geopolitical standing in the competition with other imperial powers. The U.S. would therefore need to control the Pacific with naval bases on islands and submarine telegraph cables between them, and unite both U.S. coasts via a canal linking Atlantic and Pacific. In summary, the spatial perspectives and geopolitical thinking disseminated by the geographical societies helped legitimise and popularise American imperial influence in the Pacific region and beyond in the nineteenth and twentieth centuries.