The phase stability of epitaxial KTaxNb1−xO3 (0 ≤ x ≤ 1) thin films, with compositions over the entire solid solution range, was investigated. KTaxNb1−xO3 thin films were deposited on (100) MgAl2O4 substrates by metalorganic chemical vapor deposition. Films with compositions x ≤ 0.30 were orthorhombic, as determined by x-ray diffraction. Dielectric measurements at room temperature indicated the presence of morphotropic phase boundaries at x = 0.30 and at x = 0.74. Temperature-dependent measurements of the dielectric constant for KNbO3 from 80 to 800 K indicated three structural phase transitions at 710, 520, and 240 K. For intermediate compositions, a decrease in the Curie and tetragonal–orthorhombic transition temperatures was observed with increasing Ta atomic percent, similar to the bulk phase equilibrium. In contrast to bulk materials, an increase in the orthorhombic–rhombohedral transition temperature with increasing x was observed for the films, resulting in the stabilization of a rhombohedral phase at room temperature for compositions 0.45 ≤ x ≤ 0.73. Differences between the phase stability for the thin films and bulk were attributed to lattice misfit strain.