The international community is increasingly aware of the negative impacts of child marriage on a wide range of development outcomes. Ending child marriage is now part of the Sustainable Development Goals. Yet investments to end the practice remain limited across the globe and 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 associated with the impacts are estimated for several of the impacts. When taken together across countries, the costs of child marriage are very high. They suggest that investing to end child marriage is not only the right thing to do, but also makes sense economically.