Two-dimensional (2D) Bi₂O₂Se nanosheets, as an emerging ternary layered semiconductor, exhibit promising potential for photodetection owing to their moderate bandgap, high carrier mobility, and excellent environmental stability. However, their intrinsically high carrier concentration typically results in elevated dark currents and sluggish response speeds, thereby limiting further performance enhancement. To synergistically optimize both the response speed and sensitivity, we fabricated n-type Bi₂O₂Se nanosheets via chemical vapor deposition (CVD) and integrated them into a Bi₂O₂Se/InSe semi-vertical heterojunction photodetector featuring a single-sided depletion region. Benefitting from the type-II band alignment and the graphene bottom electrode, photogenerated carriers are efficiently separated and rapidly extracted. This design simultaneously shortens the carrier transit time and suppresses recombination, enabling the device to achieve high sensitivity (responsivity R of 0.47 A/W, detectivity D* of 3.21 × 10¹² Jones, external quantum efficiency (EQE) of 166.09%) while maintaining ultrafast response characteristics (rise/fall times of 48.5/41.7 μs). The photodetector exhibits broadband self-powered operation across ultraviolet (UV) to near-infrared wavelengths (300–1050 nm). These results highlight the significant potential of Bi₂O₂Se/InSe semi-vertical heterojunctions for high-performance, low-power, self-powered broadband photodetectors spanning the UV–visible–near infrared ray (UV–Vis–NIR) spectrum.