Since its creation as a port of trade in the sixteenth century, Acapulco has been built, destroyed, and recreated by the ebb and flow of people crossing its bay. Having linked Asia with the Americas for nearly three centuries, the seaport eventually lost all commercial centrality during the nineteenth century. Its trans-Pacific horizons narrowed and disappeared altogether as Mexico’s Pacific coast pulled away from old commercial routes between Europe and Asia, and began to gravitate towards new markets, coasting trade, and mobility on the west coast of North America. “From Pacific Gateway to Tourist City” chronicles the drastic transformation of Acapulco and the Mexican seaside from 1849 to 1970, that is, the period running from the California Gold Rush up until the dawn of mass tourism. Drawing from a wide array of archives in Mexico, California, and the United States, this project shows how Acapulco and the Mexican seaside underwent profound urban changes when the eastern corner of the Pacific became the North American West, a historic map that Acapulco helped create and reconnect through its own reinvention. In this story of relentless change, Acapulco became a pioneer station during the 1849 Gold Rush, and for the rest of the century served as a hub of black markets, revolutions, and steamship traffic. Land and air transportation began to integrate many seaside economies into the West Coast as the urban heartland of Mexico and the United States began to connect and develop massive consumer markets. Roads arrived in 1927, connecting the Pacific port to Mexico City. Henceforth, the last remains of this historic map disappeared. One of the oldest Pacific seaports in the Americas, as a result, turned into one of the first boomtowns of mass tourism in North America. Why this came to be is the question animating this dissertation, while the history of this Pacific port city— its decline and reinvention, boom and bust—is the subject of this study. Acapulco’s particular transition- from a commercial seaport to a transport hub to a boomtown of mass tourism- reflects a sea change in the economic relationship drawn not only between North America, the Mexican heartland, and the Pacific World, but also between port cities and their relationship with the sea. Acapulco, after all, became a full-fledged tourist city when its age-old port disappeared and the beach displaced the bay from its central role in the urban economy, in a similar way as the story of so many other seaports. In a history of great leaps and discontinuities, of sometimes violent displacements –-of groups, families, and individuals— that changed the make-up of classes in the port, this study shows how shifting generations of merchants, revolutionaries, businessmen, boosters, and political actors in Acapulco reacted to these profound changes. The study explores their attempts to revive, transform, and adapt Acapulco’s economy form part of the larger story. Two intertwined stories are told in this dissertation: the deeply local seaport life, the individual stories, the small connections of urban, cultural, and economic life in the changing space of the port city; and the global, the shifting world-economies, the macro picture of popular upheavals and revolutions in transportation, mobility, and communications. Both stories are woven together into a comprehensive story of urban change which, in its most essential aspects, is about displacement and reinvention in the city of Acapulco. One of the central arguments in this dissertation is that economic decline, at a regional level, ultimately gave birth to the Mexican seaside in the twentieth century. As social practices on the seashore changed, attitudes towards the sea and the ocean began to shift. Alongside this process, seaside aesthetics were culturally reinterpreted as a form of lucrative natural beauty for consumption purposes. But these subtle changes were only one of the driving forces behind the rise of the Mexican seaside in Acapulco.. Economic decline also had its part. Beach tourism in Mexico was ultimately born in Acapulco, the epicenter of seaside change. It sprung from the struggles to revive commerce and find ways out of economic decline in Acapulco and along the Pacific Coast. The resort town surged forth in the 1920s and deeply transformed Acapulco’s Santa Lucía Bay. Tourism kept spreading during the postwar years. Its instant success quickly swept across disconnected port cities and economically depressed economies. It then spread to the rest of the Pacific and, many decades later, to the Gulf Coast and the Caribbean in Mexico. In the end, Acapulco’s story of urban change in the nineteenth and twentieth centuries neatly encapsulates this momentous displacement of geographies, social mores, and economic activities along the coast in Mexico. Acapulco’s story answers new questions on the dynamics of urban growth, connectedness, and the politics of mobility in port cities in general. Engaging such questions, this dissertation offers an alternative story of how, at the edge of coastal frontiers, new cities have emerged and grown from old trading seaports. It examines a central paradox in the broader urban literature: the decline of port towns gave way to the rise of coastal cities, which resulted in most port towns losing their primeval, commerce-oriented relationship with the sea. Once serving as trading outposts on coastal frontiers, my contention is that their transformation into cities, with their growing urban markets regulating and creating new forms of mobility like mass tourism, is a story central to urban growth along the coast and the formation of modern seaside economies.