Abstract Potassium–sodium niobates (K x Na 1− x NbO 3 , 0 < x < 1, KNN) were hydrothermally synthesized under varying alkaline ratios (K + /Na + ), total hydroxide concentration, reaction temperature, and time. Compositional surveys were developed by using Rietveld analyses derived quantitative volume fractions. The data demonstrated that phase pure KNN synthesis can be achieved by reacting the niobium source with the hydroxide solution having 6 M total hydroxide concentration, cation ratio (K + /Na + ) of above 6 at temperatures ≥200°C for 24 h. Dissolution–precipitation events through intermediate products including hexaniobates were postulated as a plausible formation mechanism. It was shown also that the single‐phase KNN approaching the morphotropic phase boundary (MPB) could be obtained by further incorporation of sodium ions into the crystal via post‐annealing at 800°C/2 h, following the hydrothermal synthesis.