As an environmentally friendly and chemically robust crystal, LiInSe₂ (LISe) exhibits promising optoelectronic properties. Here, an ultrasensitive metal–semiconductor–metal (MSM) structured photodetector based on a high-quality LISe crystal was systemically investigated. Remarkably, the LISe based photodetectors exhibit an outstanding photosensitivity with a high on/off current ratio (6.80 × 10⁴), fast photoresponse (t_rise = 180 μs and t_decay = 200 μs), and high detectivity (2.16 × 10¹² Jones). This work may lay the foundation and ignite future research interest on chalcogenide crystals in photodetectors.