However, such photodetectors are still limited in practical application due to the lack of broadband absorption and slow response. Layered 2D materials of nano-thickness have been shown to possess good optoelectronic characteristics leading to great potential in overcoming the major obstacles of effective broadband and sensitivity. [5,6] For example, graphene as the first reported 2D material has drawn much attention due to its unique electrical and mechanical properties. [7,8] However, graphene exhibits major limitations on the optoelectronic application, such as the weak light absorbance, large dark current, and fast carrier recombination due to zero bandgap. [9,10] There are many alternative layered 2D materials that have been tested and reported since, including transition-metal dichalcogenides (TMDC), [11-15] indium selenide (InSe), [16] hexagonal boron nitride (h-BN), [17] and black phosphorus. [18] In the class of layered 2D materials, TMDC holds great promise because of their direct band gap, strong absorption, and atomic-scale thickness with favorable electronic and mechanical properties. 2D transition-metal dichalcogenides have attracted significant interest in recent years due to their multiple degrees of freedom, allowing for tuning their physical properties via band engineering and dimensionality adjustment. The study of ternary 2D hafnium selenosulfide HfSSe (HSS) high-quality single crystals grown with the chemical vapor transport (CVT) technique is reported. An as-grown HSS single crystal exhibits excellent phototransistor performance from the visible to the near-infrared with outstanding stability. A giant photoresponsivity (≈6.4 × 10 4 A W −1 at 488 nm) and high specific detectivity (≈10 14 Jones) are exhibited by a device fabricated by exfoliating singlecrystal HSS of nano-thickness on a rigid Si/SiO 2 substrate. The application of HSS single crystal is extended to yield a sensible flexible photodetector of photoresponsivity up to ≈1.3 A W −1 at 980 nm. The photoresponsivity of CVT-grown HSS single crystal is significantly larger than those fabricated with other existing Hf-based chalcogenides. The results suggest that the layered multi-elemental 2D chalcogenide single crystals hold great promise for future wearable electronics and integrated optoelectronic circuits.