In the Northern Netherlands during the Eighty Years' War, there was an ambivalent attitude to the knowledge and use of Spanish. For some, a knowledge of Spanish was to be viewed with suspicion. For others, such as Constantijn Huygens (1596–1687), knowledge of the language was an essential part of being a humanist scholar. Although he had certain reservations about aspects of Spanish such as its sound, Huygens clearly recognized the cultural importance of the language. He was not only able to read works of literature by authors such as Cervantes, Quevedo and Fray Luis de Granada, and other books written in Spanish on subjects such as science, music and theology, but he was also able to use his knowledge of Spanish to publish translations of Spanish proverbs, write poetry in the language and keep abreast of cultural developments in Spain, particularly in the sphere of music. However, Huygens’ interest in Spanish was not merely cultural; it was also diplomatic and political. He was the secretary to two stadholders and clearly saw it as advantageous to himself and indeed to the United Provinces as a whole for him to be able to acquire and use Spanish. In this regard, his use of the language included breaking a Spanish code at the siege of Den Bosch in 1629 and corresponding with a number of important diplomatic figures in Spanish, such as the secretary to the Portuguese mission to the United Provinces. He also encouraged his children to learn Spanish, and his son Lodewijck was able to use his knowledge of the language to act as an interpreter for a diplomatic mission to the Spanish court in 1660. In short, this article provides an interesting case study of a Dutchman, who actively engaged with the Spanish language both during and after the Eighty Years' War. It demonstrates that although Huygens had some very practical reasons for acquiring a knowledge of Spanish, he also valued it as the language in which great works of literature and scholarship were written. This in turn allowed him to disseminate knowledge of the culture of Spain in the Northern Netherlands at a time when many were deeply suspicious of it as a result of the hostilities between the two countries.