This research was conducted to determine the effect of potassium (K) and cultivar on important quality traits of tomato (Solanum lycopersicum L.), including reducing sugar, titratable acidity, and vitamin E content. Tomato plants were grown in a soilless system. Three K levels (low, middle, and high equal to 150, 300, and 450 mg·L⁻¹ in the nutrient solution, respectively) and three cultivars (SVR, Kabiria, and Esperanza) were compared. Among cultivars, Kabiria, which is characterized by smaller fruits, showed 23% higher total soluble solids (TSS) than the average of the other cultivars. ‘Kabiria’ also showed a total tocopherol (vitamin E) content (18.5 mg·kg⁻¹), markedly higher than SVR and Esperanza cultivars (12.2 and 10.3 mg·kg⁻¹, respectively). Increased K levels in the nutrient solution resulted in increased contents of TSS, reducing sugar contents and titratable acidity in tomato fruits. Also, the vitamin E content of tomato fruits was significantly affected by differing K concentrations in the nutrient solution.