Vegetable African nightshades (Solanum L. Section Solanum) are important in food and financial security. Their yields are limited by low soil fertility. Effect of varied rates of potassium on performance and profitability of these vegetables was determined in a 2 x 4 x 4 factorial experiment (RCBD) conducted at Malava and Bukura areas in Western Kenya. The main factors were site (two agro ecological zones), genotype and potassium. Each of four species representing genotype; Solanum villosum subsp. Villosum, Solanum villosum Miller subsp. Miniatum, Solanum scabrum Miller and Solanum sarrachoides Sendtner received 0, 33, 66 and 99 kg K ha-1 (KCl 60 % K) hence sixteen treatment combinations replicated three times per site. Low soil exchangeable K and organic carbon were found in Bukura (0.1 Cmol K kg-1) and Malava (0.2 Cmol K kg-1). Genotype interacted significantly (P≤0.001) with potassium in affecting fresh leaf yields. Overall, mean yields of Solanum villosum subsp. Villosum (4.89 tons ha-1), Solanum villosum Miller subsp. Miniatum, (4.58 tons ha-1) and Solanum scabrum Miller (11.46 tons ha-1) receiving 66 kg K ha-1 were significantly higher (P≤0.05) than control (3.88, 3.04 and 5.27 tons ha-1 respectively). Solanum scabrum Miller had higher Benefit/Cost ratios (61.16, 53.55) at 33 kg K ha-1 compared to Solanum villosum subsp. Villosum (7.18, 9.57) and Solanum villosum Miller subsp. Miniatum (10.93, 2.75) in Malava and Bukura respectively. Solanum sarrachoides Sendtner had low Benefit/Cost ratios (-5.46 to 1.54). Effect of manure and inorganic K sources on the yield of vegetable African nightshades should be investigated.