Hydroelectric power plants account for 25% of West Africa's total installed electricity generation capacity. In general, electricity generated by these plants is seasonal, and intra-annual fluctuations have a considerable impact on electricity supply and cross-border electricity trade. This study soft-links a global hydrologic model to a multi-region capacity expansion and planning model for the West Africa Power Pool (WAPP) to examine the effects of seasonality in hydropower electricity generation on electricity trade as well as the economic benefits of unconstrained cross-border electricity trade. Transitioning from a rainy to a dry season decreases hydroelectricity generation by an average of 40% in a typical year. Between 2018 and 2050, satisfying the region's electricity demand will require significant capital investment; nevertheless, extending from current bilateral electricity trade agreements to a fully competitive cross-border electricity trade will result in net annual savings of $3 billion in 2015 dollars. To achieve year-round energy security, countries should plan to diversify their energy sources to ensure that supply reliability can be maintained in the event of lower supply during the dry season. This study represents a novel examination of seasonality impacts on hydropower modeling and may serve as a test case to inform future modeling efforts in other regions.