The legal recognition of civil unions between same-sex partners, in May 2016, could be defined as a revolution for the system of legal norms regarding the “heterosexual family-centric-system” in Italy. Using official data on the 17,341 people resulting in a same-sex civil union collected by the Italian National Institute of Statistics in the years 2016–2018, this paper analysed the relationship between religious secularization and the diffusion of same-sex civil unions at the regional level in Italy. In particular, an indicator of the incidence of civilly united individuals over total population has been regressed on the rate of heterosexual marriages celebrated according to the civil rite. According to abundant literature, the latter variable could be interpreted as a proxy of secularization. The results indicate that less secularized regions are also those where less civil unions have been celebrated. The results are confirmed also when an instrumental variable approach is implemented. Studying this topic in Italy is particularly interesting since the country has been defined as the least secularized among economically developed nations.