Tis paper explores the emergence of the notion of sexuality in late-nineteenth-century medicine by analysing various writings of Paolo Mantegazza such as Fisiologia dell'amore (1873), Elementi di igiene (1875), and Gli amori degli uomini (1885). With this aim in mind, this study discusses first Mantegazza's broader project on the physiology of emotions in order to explain in the second part the role of his contribution to the idea of love as the foundation of sexual medicine. Tus, love is an affection that loses its religious connotation in order to be studied like the other forces of nature according to Charles Darwin's theory of natural selection. Furthermore, Mantegazza's concept of love was not exclusively focused on the anatomical structure of internal and external genital organs, what we understood as sex, but it also included the psychological dimension of the individual,and thus it was understood as a matter of choice and taste, i.e. as sexuality. In this way, Mantegazza's scientific research reveals the shiſt from the old category of love to the emergent concept of sexuality, which transformed the sexual behaviour into an object under the administration and management of medical discourse. Finally, this article shows how love in the time of Darwinism opened the way to the modern category of sexuality as a singular experience considered to be the most important expression of our personality in contemporary society.