Tetragonal (00l)-oriented epitaxial (Bi,K)TiO₃ films were grown at 240 °C on (100) _cSrRuO₃//(100)SrTiO₃ substrates by the hydrothermal method. KOH aqueous solutions and Bi(NO₃)₃ · 5H₂O and TiO₂ powders were used as the starting materials. Film thickness was controlled from 33 to 1200 nm by changing the deposition time, and the Bi/(Bi+K) ratio in the A-site of perovskite ABO₃ was almost constant for all film thicknesses. Polar-axis (00l)-oriented epitaxial (Bi,K)TiO₃ films were obtained without a secondary phase and/or other orientation for all thickness ranges. Large ferroelectricity with the remanent polarization ( P_r) of about 84 µC/cm², comparable to previously reported lead-based ferroelectric films, was observed for (Bi,K)TiO₃ films down to 33 nm in thickness. On the other hand, E_c increased with decreasing film thickness, but did not show strong film thickness dependence like other perovskite ferroelectric films. These data are very useful for understanding the degradation mechanism of ferroelectric thin films.