A body of literature on Cold War international history has studied the influence of modernisation theory in United States foreign relations with its authoritarian allies in the Third World during the 1950s and 1960s. However, this area of research has been much less interested in those Washington-friendlydictatorships that, as in the case of Francisco Franco’s regime in Spain, do not fit into the Third World analytical framework. This analysis assesses the contribution of modernisations doctrine principles to American foreign policy towards the Spanish dictatorship in the 1960s and considers how this theory provided the conceptual tools to justify the American collaboration with the Franco regime in the name of development, security, and long-term democratisation. It examines the role of modernisation as, on one hand, an interpretative framework for Spain’s economic and social evolution during that decade and, on the other, an instument of political legitimisation serving American strategic interests. In this way, this work sheds light on the ideological and intellectual underpinnings of the American alliance with the Franco dictatorship in a period of great challenges and transformations in Spain.