From 17 to 22 October 1955, Madrid hosted the UNESCO Festival of Science. In the early years of the Cold War, in a dictatorial country that had recently been admitted into the international community, the festival aimed to spread science to the public through displays of scientific instruments, public lectures, book exhibitions, science writers professional associations, and debates about the use of different media. In this context, foreign visitors, many of whom came from liberal democracies, seemed comfortable in the capital of a country ruled by a dictatorship that had survived after the defeat of fascism in the Second World War and was struggling to gain foreign recognition after years of isolation. This article analyzes the political role of science popularization in Madrid at that time. It approaches the apparently puzzling marriage between UNESCO’s international agenda for peace and democracy and the interests of the Francoist elites. Shared views of technocratic modernity, the fight against communism, and a diplomacy that served Spanish nationalism, paved the way for the alliance.