<p>Urbanization constitutes the most important transformation in the population geography of developing countries but evidence on the underlying spatial patterns of internal migration is limited. We investigate in an internationally comparative perspective whether out-migration diffuses over the course of urbanization from larger and denser populated settlements to smaller and sparser ones, and whether migrants head to increasingly smaller urban destinations, as conjectured by the mobility transition hypothesis. Relying on population censuses fielded between 1970 and 2017 and internationally consistent definitions of the spatial extent of urban agglomerations, we estimate different types of in- and out-migration across more than 8000 distinct rural and urban settlements in 41 countries. We use spatial econometrics to estimate migration gradients along the rural-urban continuum and investigate how these spatial patterns evolve during urbanization. Results confirm the conjectured changes in the sending and destination areas of migration. At the onset of urbanization, total out-migration is the highest in large cities. The rural exodus emerges in proximate and dense rural settlements and diffuses into the remote and sparsely populated countryside. While larger cities constitute the main migrant destinations in early stages of urbanization, inter-city and rural-to-urban migrants increasingly head towards intermediate-sized and small cities in later stages. These insights are discussed with reference to the spatial diffusion of development. We conclude that the mobility transition hypothesis remains useful for informing migration trends, urban population projections and regional development policies.</p>