The mid-infrared (mid-IR) second-order optical nonlinearity of the barium titanate (BTO) thin films was characterized by second harmonic generation (SHG). The epitaxial BTO thin films were grown on strontium titanate substrates by pulsed-laser deposition. From the azimuthal-dependent polarized SHG measurements, the tensorial optical nonlinear coefficients, d_ij, and ferroelectric domain fraction ratio, δA_Y/δA_z, were resolved. Strong SHG signals were obtained at the pumping laser wavelength λ between 3.0 and 3.6 μm. The SHG intensity was linearly dependent upon the square of the pumping laser power. The broadband mid-IR optical nonlinearity enables BTO thin films for applications in chip-scale quantum optics and nonlinear integrated photonic circuits.