Maria de Lourdes Pintasilgo, the only woman, to this day, to serve as Prime Minister in Portugal, played a prominent role in major international forums. This article analyzes her intervention as Portugal’s Ambassador to UNESCO between the years 1974 and 1979. Based on reference bibliographies, archival documents, and periodicals from that time, it seeks to investigate the decision-making process that led to Portugal’s request to rejoin UNESCO; the reasons behind the choice of Maria de Lourdes Pintasilgo for the position of ambassador; and to identify her main areas of activity aimed at ensuring Portugal’s reintegration into the organization, following the voluntary withdrawal in 1971, by decision of the Portuguese Government.