Tidal flats are a highly productive coastal ecosystem with a species diversity thought to rival that of tropical rain forests, yet we know remarkably little about their distribution and status. In East Asia, tidal wetlands are the frontline ecosystem protecting a coastal population of more than 150 million people from storms and sea-level rise. Unprecedented coastal development, particularly in China and South Korea, has led to growing concern about status of coastal wetlands in the region. We developed a new remote sensing method to assess change over >4000 kilometers of the Yellow Sea coastline and discovered vast losses of the region’s principal coastal ecosystem, tidal flats, driven primarily by urban, industrial, and agricultural land reclamations. Our methods provide a framework for assessing change of intertidal ecosystems and can be applied to any geographic region with adequate satellite data.