This dissertation unites marine environmental history, transnational history, and foreign relations history to explore how marine space was renegotiated after the Russian Empire sold Alaska to the United States in 1867. The title comes from a popular, humorous moniker—“Walrussia”—that American newspaper reporters applied to Alaska after its purchase. This moniker highlights the ways in which both Russian and American colonizers envisioned and experienced Alaska as a marine space. As Russia, the United States, Canada, and Japan renegotiated the boundaries of their maritime sovereignty following the geopolitical shift of the transfer of Alaska, northern fur seals arose as coveted mobile resources and generators of biopower for the region. This dissertation argues that the fur seal industry—begun by Russians and transferred to Americans by way of Aleut Natives—organized transnational relations in the North Pacific following the transfer of Alaska. When Americans and Canadians made the prudent ecological decision to protect North American fur seals, the problem shifted westward, and the Russian navy began to collide with foreign seal hunters. The diplomatic incidents that ensued forced the empires of the North Pacific to carefully delimit maritime boundaries. Northern fur seals were the first nonhuman animals to be protected by a multinational treaty, the 1911 North Pacific Fur Seal Convention, due to their profitability and the diplomatic conflict they engendered. The convention also contributed to the broader enclosure of the free sea envisioned by many Western thinkers since Hugo Grotius. In 1609, Grotius wrote "The Free Sea," a text that would become the Western standard for delineating marine space and rights in the absence of international law. The convention that determined the fate of seals ultimately concluded that Grotius was right about freedom of navigation but wrong about the inexhaustibility of resources. By adding Russian archival documents to traditional English-language histories of the fur seal crisis, this dissertation tells a robust, transnational story that reflects the mobile nature of the fur seal. The project draws on thirteen Russian, American, and Canadian archives to contribute to the subfields of maritime, environmental, transnational, diplomatic, spatial, and commodities histories.