Thin films of BiFeO₃ (BFO) were grown on a LaNiO₃ buffered glass substrate by RF magnetron sputtering. The deposition parameters were tailored for the films to exhibit either n-type or p-type conductivity, which allowed the fabrication of a BFO p–n junction for photocatalytic applications. Both p/n-type films contained oxygen vacancies with the atomic fraction being 7.7% and 2.0%, respectively. The p-type conductivity was correlated to the positively charged oxygen vacancies, which occurred in large numbers in the p-type films due to charge compensation for a higher Fe²⁺/Fe³⁺ ratio. In contrast, more oxygen vacancies in the n-type films were neutral oxygen vacancies, which were shallow electron donors. The n-type films showed a bandgap of 2.57 eV, which was slightly larger than that of the p-type films (2.50 eV). The band alignment between the p/n-type films was established based on the results of ultraviolet photoelectron spectroscopy. The Fermi level of both p/n-type films was close to the middle of the bandgap as a result of low carrier concentrations, which were consistent with the carrier concentrations calculated from the slope of the Mott–Schottky plot. The BFO p–n junction allowed a fast separation of photo-generated charge carriers as confirmed by the observation of a great increase in photocurrent, which led to a great improvement in photodegradation of methylene blue (MB). The BFO p–n junction could degrade 95.5% MB in 120 min (10 × 10 mm² film in 20 mL of 10 mg L⁻¹ MB) and the degradation efficiency remained above 90.3% after five cycles of reuse.