Soils of southwestern Nigeria have been shown to contain substantial concentration of sodium (Na). Sodium has almost identical chemical properties with potassium (K) in the soil but functions of Na in the nutrition of higher plants are not well documented. This study examined the single effects of sodium (Na) and potassium (K), and their interactions on growth, yield, nutrient content and nutritional quality of tomato (Lycopersicon lycopersicum (L.) Karst), with a view to establishing the optimal level of Na and K that will promote good growth and yield. It also examined an appropriate Na : K ratio that may be required in fertilizer formulation to enhance and sustain growth of the tomato plant. The greenhouse study was a 6 x 4 factorial, arranged into a Randomized Complete Block Design (RCBD) and replicated three times. Sodium at 0, 2, 4, 8, 16 and 32 mg Na kg-1 soil and K at 0, 32, 64 and 128 mg K kg-1 soil were applied as NaCl and KCl, respectively. In the field, Na and K treatments were based on the results obtained for greenhouse study. There were four levels of Na and three levels of K arranged in a RCBD and replicated three times. Records of the number of leaves, branches, flowers and fruits yields were taken. Water and nutrient contents of tomato plants were determined. Citric acid, pH and electrical conductivity of the tomato fruit juice were also determined. Results of the greenhouse study indicated that fruit yields, shoot-N and -Ca, and citric acid contents of the fruit juice improved significantly at 2 to 4 mg Na kg-1 soil but decreased at 16 to 32 mg Na kg-1 soil. The sodium concentration of the tomato plant was markedly increased at increasing levels of sodium application without an adverse effect on growth. The addition of Na up to 4 mgkg-1 soil to K up to 64 mgkg-1 soil produce optimal leaf and fruit yields, plant water content, and citric acid content of fruit juice. However, it was observed that at mg K 128 kg-1 soil, the addition of 32 mg Na kg-1 soil reduced the depressing effect of high K at 128 mgkg-1 soil on the growth. The number of leaves correlated positively with soil-Na (r = 0.51, p < 0.05) but negatively with soil-K (r = -0.07, p < 0.05) while the fruit yields correlated positively with the number of leaves (r = 0.65, p < 0.01) and critic acid content ( r = 0.72, p < 0.01). In the field, the number of flowers, fruit weight and the citric acid content increased at 4 up to 16 mg Na kg-1 soil and 64 mg K kg-1 soil. Shoot-N and -Ca, root-P and –Mg, and fruit-K were also increased significantly at the above stated Na and K levels. Fruit weight correlated positively with the number of leaves ( r = 0.92, p < 0.01), shoot/root ratio ( r = 0.91, p < 0.01) and shoot-N ( r = 0.78, p < 0.01). The study concluded that Na is necessary for the growth of tomato. It also has a potential of playing an essential role in the nutrition of tomato plants, particularly when combined with K at Na : K ratio of 1 : 4 up to 1 : 8. This role of Na could further be pronounced in seasons and areas where soil water may be limiting, as Na was observed to have played a role in water retention in the vegetative growth of the plant. The tomato could probably be classified as a medium Na-tolerant plant.