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Obtaining p-type wide-bandgap semiconductors with a bandgap >3.5 eV is still challenging. Here, p–n junction devices based on wide-bandgap (≥4 eV) p-type MnO quantum dots (QDs) and n-type Si-doped GaN are fabricated. The p-MnO QDs are synthesized by cost-effective femtosecond laser ablation in liquid. A simple spray-coating method is used for fabricating the p-MnO/n-GaN-based solar-blind deep UV (DUV) photodetector. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy reveal the MnO QD crystal structure. X-ray photoelectron microscopy analysis reveals good band alignment between p-MnO QDs and n-GaN, demonstrating the (type-II) staggered band alignment p–n heterojunction-based device. Electrical and photocurrent measurements show a high photocurrent response with a low dark current, while superior photo-responsivity (∼2530 mA/W) is achieved, along with self-powered and visible-blind characteristics (265 nm cutoff), demonstrating a high-performance DUV device with high detection limit for low light level applications. This study provides insights into the potential of p-type MnO QDs for III-nitride p–n junction DUV devices.