The use of piezoelectric internal field to assist in photocatalysis is important for inexpensive and high-efficiency catalytic techniques. In this study, novel Pt/BaTiO₃ heterojunctions were prepared via decoration of Pt nanoparticles (PtNPs) on the surface of piezoelectric BaTiO₃ nanocubes through a facile photoreduction process. The photocatalytic, piezo-catalytic, and piezo-photocatalytic activities of the Pt/BaTiO₃ heterojunctions for methyl orange (MO) degradation were investigated under ultrasonic excitation and/or full-spectrum light irradiation. The weight percentage of PtNPs, piezoelectric internal field of BaTiO₃, and plasmonic effect have been proven important for the photocatalytic activity of the heterojunctions. The Pt_0.25/BaTiO₃ with 0.25 wt% PtNPs exhibited an optimum photocatalytic degradation performance of 92.5% for MO in 50 min under full-spectrum light and ultrasonic co-excitation, and this value was about 1.35 times higher than the degradation rate under full-spectrum light irradiation alone. Well-dispersed PtNPs in the Pt/BaTiO₃ heterojunction can be excited by incident light to generate an electromagnetic field caused by the surface plasmon resonance (SPR) effect, which drives the collective oscillation of electrons and generate hot electron–hole pairs. The piezoelectric internal field generated in BaTiO₃ due to ultrasonic excitation can improve the separation efficiency of SPR-induced hot charge carriers and contribute to the production of highly reactive oxidation radicals, thereby enhancing the catalytic capability of the heterojunction to oxidize organic dyes. This study introduces the piezoelectric effect of BaTiO₃ into plasma photocatalysis to enhance the photocatalytic activity of Pt_x/BaTiO₃ heterojunction, and this effect can be extended to other piezoelectric material systems to provide an effective technology for environmental purification.