AbstractAs a novel V‐VI ternary topological insulator, Sb2Te2Se, has recently garnered great attention that has motivated both theoretical and experimental investigations. Numerous simulations have proved that 2D ternary Sb2Te2Se is a promising candidate for various optoelectronic devices. However, there are only a few experimental reports focusing on 2D Sb2Te2Se‐based devices due to the lack of a facile synthetic approach. Herein, few‐layered 2D Sb2Te2Se nanosheets are prepared by efficient liquid phase exfoliation and their photoresponse performance is systematically investigated using a photoelectrochemical type photodetector. As revealed, the 2D Sb2Te2Se‐based photodetector exhibits a significant photoresponse in alkaline electrolytes with excellent long‐term cycling stability (decrement of 3.3% during 15000 cycles). In addition, a wide spectral response ranging from 365 to 700 nm could be achieved, indicating great potential for broad‐band photodetection. With this initial exploration work, it is anticipated that 2D ternary Sb2Te2Se materials can be developed for future optoelectronic devices with high performance.