While previous studies have shown that nonmarital cohabitation is socially diffused, few have addressed the spatial dimension of this diffusion process. To our knowledge, no studies exist on this topic for Belgium. This article examines the spatial dimension of this demographic change in Belgium. This study aims to answer the following questions: Is there a process of spatial diffusion of nonmarital cohabitation in Belgium? What is its spatial pattern? In which regions dos nonmarital cohabitation increase first, and which are resistant to this demographic change? How has this diffusion taken place geographically? We use data from the Belgian National Register. It makes it possible to achieve analysis at a detailed geographical level (the municipality) and a large time coverage (1991-2015). We use thematic cartography to reveal the spatial pattern of diffusion of nonmarital cohabitation in Belgium. There is a spatial diffusion of nonmarital cohabitation in Belgium. It is similar to that of the fertility decline during the first demographic transition. This process has occurred through expansion diffusion (by geographic proximity) and hierarchical diffusion (through the urban hierarchy). The article highlights the importance of investigating nonmarital cohabitation from a spatial and temporal perspective. It describes the spatial pattern of the spread of nonmarital cohabitation in Belgium, a truly original result in the literature. We have also clearly identified the importance of the hierarchical aspect. To our knowledge, this has never been done before in the literature on nonmarital cohabitation. Our results have important implications for statistical modelling of this process.