A salient issue facing contemporary urban development in many countries is that the physical areas of major cities are growing at a faster rate than their populations. The popularity of the green belt concept among advocates is that it can effectively counter urban sprawl while safeguarding the countryside from urban development. This paper is intended to measure the efficacy of the green belt in preventing urban sprawl through an international comparative study in three cities of different sizes, and which have experienced different urban growth pressures, namely Frankfurt am Main (Germany), London (UK), and Seoul (South Korea). The study adopts the urban sprawl measurement methodological framework defined by Jaeger et al. to process GHSL data in order to examine the urban sprawl index in the three case study cities. This quantitative evidence-based comparative study demonstrates that the designation of green belts has failed to prevent urban sprawl both within urban centers and at a wider regional level.