BACKGROUND Previous studies have shown that nonmarital cohabitation is socially diffused. However, to our knowledge, no studies exist on spatial aspects of the diffusion. This article examines the spatial diffusion process of nonmarital cohabitation in Belgium. OBJECTIVE This study aims to answer the following questions: What is the spatial diffusion pattern of nonmarital cohabitation in Belgium? In which areas did nonmarital cohabitation increase first, and which areas are resistant to this demographic change? How has this diffusion taken place geographically? METHODS We use data from the Belgian National Register, which makes it possible to achieve analysis at a detailed geographical level (the municipality) and covering a long period of time (1991–2015). We use diachronic cartography to reveal the spatial patterns of diffusion of nonmarital cohabitation in Belgium. RESULTS We observed that spatial diffusion of nonmarital cohabitation in Belgium is similar to the spread pattern of fertility decline in the first demographic transition. This observed spatial pattern suggests to some degree that this process may have occurred by geographic proximity and through the urban hierarchy. CONTRIBUTION 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. To our knowledge, this has never been done before in the existing literature on nonmarital cohabitation. The results highlight a possible diffusion through the urban hierarchy, even if the influence of contextual conditions on diffusion within municipalities cannot be excluded. Our results, although descriptive, could have important implications for future statistical modelling of the diffusion process.