East–Southeast Asia is currently one of the fastest urbanizing regions in the world, with countries such as China climbing from 20 to50%urbanized in just a few decades. By 2050, these countries are pro- jected to add 1 billion people, with90%of that growth occurring in cities. This population shift paral- lels an equally astounding amount of built-up land expansion. However, spatially-and temporally- detailed information on regional-scale changes in urban land or population distribution do not exist; previous efforts have been either sample-based, focused on one country, or drawn conclusions from datasets with substantial temporal/spatial mismatch and variability in urban definitions. Using con- sistent methodology, satellite imagery and census data for >1000 agglomerations in the East–South- east Asian region, weshow that urban land increased>22%between 2000 and 2010 (from 155 000 to 189 000km2), an amount equivalent to the area of Taiwan, while urban populations climbed>31% (from 738 to 969 million). Although urban land expanded at unprecedented rates, urban populations grew more rapidly, resulting in increasing densities for the majority of urban agglomerations, includ- ing those in both more developed (Japan, South Korea) and industrializing nations (China, Vietnam, Indonesia). This result contrasts previous sample-based studies, which conclude that cities are uni- versally declining in density. The patterns and rates of change uncovered by these datasets provide a unique record of the massive urban transition currently underway in East–Southeast Asia that is impacting local-regional climate, pollution levels, water quality/availability, arable land, as well as the livelihoods and vulnerability of populations in the region.