This paper assesses the economic implications of adopting a common external tariff (CET) for Africa as a next step toward establishing an African Customs Union following full implementation of the African Continental Free Trade Area (AfCFTA). Using the MIRAGE-Power computable general equilibrium (CGE) model, it examines the impact of extending the CET structures of the East African Community (EAC) and the Economic Community of West African States (ECOWAS) to all African countries, and explores optimal CET structures based on these models through a grid search procedure. The analysis focuses on ensuring World Trade Organization (WTO) compliance—by keeping post-implementation tariffs on imports from non-African partners no higher than before—and on identifying scenarios that best promote Africa’s gross domestic product (GDP), welfare, or intra-African trade. Results indicate that adopting either the EAC or ECOWAS CET structure would make Africa more protectionist overall, reducing total trade with the rest of the world but increasing intra-African trade, with varying effects across countries. Two optimal WTO-compliant CET scenarios show potential for improving GDP, welfare, and total trade, though with some loss in intra-African trade gains achieved under AfCFTA, underscoring the need for balanced compromises in continent-wide tariff harmonization.