The continued improvement in the yield of cowpea for the semi-arid regions will record rapid progress if the genetic basis for yield under different moisture regimes is understood. This study investigated genetic combining abilities for yield components, earliness to flower and biomass production among diallel hybrids derived from nine cowpea genotypes. Hybrids and their parents were evaluated under adequate soil moisture conditions and also under conditions of soil moisture stress during the first 30 days of growth. Soil moisture stress significantly reduced grain yield, biomass production, and the number of pods per plant. Grain size increased under moisture stress. Genetic control for grain size, number of seeds per pod and days to flowering was dominated by additive genetic effects. For grain yield, both additive and non-additive genetic effects were dominant, but the greater influence of non-additive effects was observed under both adequate soil moisture and moisture stress conditions. The genetic control of yield under contrasting moisture conditions showed that grain yield improvement will be more challenging for drought prone regions compared with regions with adequate soil moisture. Nonetheless, careful selection of genotypes that have complementary but different alleles for yield might permit yield improvement through recurrent selection systems.