Two-dimensional (2D) self-driven photodetectors have attracted extensive attention due to the fast photoresponse, low energy consumption and high flexibility. Recently, the metal–semiconductor-metal (MSM) structures with asymmetrical 2D semiconductor channel are exploited for the self-driven photodetection. However, the mechanism is still in dispute and the channel has not yet been forward to monolayer. Here, a self-driven MSM photodetector is fabricated with the directly-grown monolayer WSe₂ which is asymmetrically patterned by the controllable scratching lithography. The self-driven behavior mainly originates from the asymmetry of both conduction channel and the Schottky junctions. With the photovoltaic effect driven by asymmetry, the device exhibits linear photoresponse with responsivity of 1.4 mA/W, detectivity of 10¹⁰ Jones, dark current level of 0.1 pA and response speed of tens of milliseconds. Moreover, performances show highly gate-tunable characteristics due to the tunable Schottky barriers. This work provides a new route towards self-driven photodetectors based on 2D materials.