The international community is increasingly aware of the negative impacts of child marriage on a wide range of development outcomes. Ending child marriage is nowpart of the Sustainable Development Goals. Yet investments to end the practice remain limited across the globe. Ethiopia recently adopted a strategy to end child marriage, and some of the projects being implemented in the country should contribute to reduce the practice child marriage. Still, more could be done. In order to inspire greater commitments towards ending child marriage, this study demonstrates the negative impacts of the practice and their associated economic costs. The study looks at five domains of impacts: (i) fertility and population growth; (ii) health, nutrition, and violence; (iii) educational attainment and learning; (iv) labor force participation and earnings; and (v) participation, decision-making, and investments. Economic costs are estimated for several of the impacts. Overall, the costs are high. They suggest that investing to end child marriage is not only the right thing to do, but also makes sense economically.