This study investigated the dissolution of niobic acid (Nb<inf>2</inf>O<inf>5</inf>·nH<inf>2</inf>O) and potassium niobate (mix of phases K<inf>4</inf>Nb<inf>6</inf>O<inf>17</inf> and KNbO<inf>3</inf>) using the polythermal method in different concentrations of a KOH solution (0–2 mol/L). The software OLI Studio Stream Analyzer and PHREEQC were used for the thermodynamic modeling. The equilibrium time of niobic acid dissolution in an alkaline medium was investigated. Aqueous niobium concentration was measured by ICP-OES and UV–vis spectroscopy methods. The measured dissolved Nb concentrations at 30 °C show deviations from predicted values, indicating the lack of niobium solubility data. The experimental results from the polythermal method evidenced that the dissolution of niobic acid increases with temperature and decreases with KOH concentration. Nevertheless, potassium niobate exhibited a complex behavior, with different phases dissolving preferentially under the studied conditions (0–1 mol/L). Our findings contribute to the understanding of niobium chemistry in alkaline medium, crucial for the development of hydrometallurgical processing of niobium primary sources, which is directly related to the efficiency of leaching processes, crystallization control, and subsequent recovery steps of secondary sources.