Bi_1−xSm_xFeO₃ thin films (x = 0, 0.05, 0.10 and 0.15) on Pt(111)/Ti/SiO₂/Si substrates were fabricated by the sol–gel process and the effect of Sm doping on their crystal structure was studied by synchrotron radiation X-ray diffraction and Raman spectroscopy. It is revealed that a phase transition from the rhombohedral to orthorhombic structure takes place with increasing Sm content, resulting in two-phase coexistence at x = 0.10, where the two phases are R3c and Pbam according to the refinement result. The phase transition can be ascribed to the difference between the smaller radius of substituted Sm³⁺ and Bi³⁺. Meanwhile, the composition-dependent dielectric, ferroelectric and piezoelectric properties were also investigated. PFM scanning and switching spectroscopy results confirmed the enhancement of the piezoresponse at x = 0.10 corresponding to the rhombohedral–orthorhombic morphotropic phase boundary (MPB) region. The ferroelectric properties of Sm-doped BiFeO₃ films were found to decrease with increasing Sm content, indicating that the extrinsic piezoelectric response contributes more to the improved piezoelectricity at the MPB.