An ultraviolet (UV) photodetector based on amorphous ZnSnO (α-ZTO) channel thin-film transistors (TFTs) with ultrahigh sensitivity was fabricated by a simple sol–gel method. The photodetectors are investigated at various intensities of 365 nm UV illumination, which exhibit ultrahigh sensitivity and responsivity. A reasonable mechanism is proposed for the superior UV detecting performance. Moreover, our device is promising for UV detection under weak UV illumination. The fabrication of the α-ZTO UV photodetector represents a significant step toward future multifunctional optoelectronic applications.