van der Waals heterostructures (vdWHs) based on two-dimensional (2D) materials without the crystal lattice matching constraint have great potential for high-performance optoelectronic devices. Herein, a WS₂/InSe vdWH photodiode is proposed and fabricated by precisely stacking InSe and WS₂ flakes through an all-dry transfer method. The WS₂/InSe vdWH forms an n—n heterojunction with strong built-in electric field due to their intrinsic n-type semiconductor characteristics and energy-band alignments with a large Fermi level offset between WS₂ and InSe. As a result, the device displays excellent photovoltaic behavior with a large open voltage of 0.47 V and a short-circuit current of 11.7 nA under 520 nm light illumination. Significantly, a fast rising/decay time of 63/76 µs, a large light on/off ratio of 10⁵, a responsivity of 61 mA/W, a high detectivity of 2.5 × 10¹¹ Jones, and a broadband photoresponse ranging from ultraviolet to near-infrared (325–980 nm) are achieved at zero bias. This study provides a strategy for developing high-performance self-powered broadband photodetectors based on 2D materials.