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Self‐driven broadband photodetectors have wide applications in the fields of biomedicine, remote sensing, rescue, and mineral exploration with advantages of energy conservation and multiband detection. However, most present broadband photodetectors are suffering from a fast degradation of photoresponsivity in ultraviolet (UV) region. To resolve it, a self‐driven broadband photodetector is proposed based on mixed‐dimensional 2D PtSe2/3D amorphous Ga2O3 (a‐Ga2O3) heterojunction considering the high UV responsivity of a‐Ga2O3 thin film. 2D PtSe2 is obtained on a‐Ga2O3 thin film by a simple selenization method directly. The responsivity of the completed device in UV region is about 14 and 172 times higher than that in visible and NIR regions, respectively. In addition, benefiting from the excellent built‐in electric field at the heterojunction and high carrier mobility of 2D PtSe2, photogenerated electron–hole pairs can be rapidly separated. As a result, its rise time (9.36 ms) and decay time (11.27 ms) are much faster than those of the current a‐Ga2O3‐based self‐driven photodetectors (≈100–1000 ms). This work provides a novel building block via a facile strategy for the further development of high‐performance, low‐cost, and energy‐efficient broadband photodetectors. [ABSTRACT FROM AUTHOR]