Nitrogen (N) metabolism in plants requires adequate potassium (K) content in the cytoplasm. However, the influence of NH4+ in the nutrient solution on plant uptake of K is controversial. To identify the effect of N form on K uptake in sweet pepper (Capsicum annum L. cv. Hazera 1195) two hydroponic experiments containing three K concentrations (0.5, 2.5, and 5.0 mM) combined with four NH4+-N to NO3--N mM ratios (0:6, 0.9:5.1, 1.8:4.2, or 3:3) at a constant total of 6 mM N were conducted during the spring-summer and autumn-winter seasons. Early fruit yield was affected by K concentration but not by N form. Replacing 0.9-1.8 mM (15-30%) of NO3--N by NH4+-N increased total fruit yield, especially when only 0.5 mM K was supplied. No significant interaction of NH4+-N and K concentration on early and total fruit yield was found in either season. The uptake rate of NH4+-N was related to its concentration and was independent of K concentration in the culture solution. The uptake rate of K was reduced as the NH4+-N concentration increased above 1 mM. Compared to sole NO3--N supply, adding 0.9-1.8 mM NH4+-N significantly increased the plant leaf phosphorus (P) and N content, but did not affect the leaf K content in the high yielding spring-summer season. It is concluded that 0.9-1.8 mM NH4+-N (15-30% of total N) in the nutrient solution could increase both the total fruit yield and fertilizer K use efficiency in sweet pepper.