Twenty nine cowpea genotypes, including four Ugandan genotypes, were evaluated for grain yield, protein stability and adaptability under diverse environments in a randomized complete block design with three replications. The analysis showed that cowpea grain yield and protein content were significantly (P < 0.01) affected by genotypes (G), environments (E), and interaction (G x E). Genotypes C2T and C2I had the highest grain yield and protein content respectively, but both were only adapted to specific environments. Genotypes C1J, C1V, C2A, C2O, and C2R were adapted to three environments with high yield which was stable. Similarly, genotypes BRS Pujante, C1J, C2Q and CIT also expressed high protein levels with high stability and wide adaptability. The study further revealed that Namulonge 2014B, Serere 2014A, Serere 2014B and Serere were the most favorable environments for obtaining high yield and protein respectively, because at these four environments, mean yield and protein were higher than the overall mean. All Brazilian genotypes had high protein levels compared to Ugandan genotypes indicating the potential of Brazilian genotypes in improving cowpea seed protein content in Uganda.