We demonstrate changes in the saturation magnetization of Pd(001) ultrathin films exhibiting ferromagnetism in Pd/SrTiO3/BaTiO3 heterostructures due to the strain effect caused by the structural phase transition of BaTiO3 substrates. When a BaTiO3 substrate with a single domain formed by voltage application transitioned from the orthorhombic phase to the tetragonal phase, the saturation magnetization of Pd decreased by about 5%. The first-principles calculations, which assumed the distortion accompanied by the orthorhombic to the tetragonal structural phase transition, indicated a decrease by 3%-4% in the magnetization, which was consistent with the experimental results.