2D ternary semiconductor single crystals, an emerging class of new materials, have attracted significant interest recently owing to their great potential for academic interest and practical application. In addition to other types of metal dichalcogenides, 2D tin dichalcogenides are also important layered compounds with similar capabilities. Yet, multi‐elemental single crystals enable to assist multiple degrees of freedom for dominant physical properties via ratio alteration. This study reports the growth of single crystals Se‐doped SnS2 or SnSSe alloys, and demonstrates their capability for the fabrication of phototransistors with high performance. Based on exfoliation from bulk high quality single crystals, this study establishes the characteristics of few‐layered SnSSe in structural, optical, and electrical properties. Moreover, few‐layered SnSSe phototransistors are fabricated on both rigid (SiO2/Si) and versatile polyethylene terephthalate substrates and their optoelectronic properties are examined. SnSSe as a phototransistor is demonstrated to exhibit a high photoresponsivity of about 6000 A W−1 with ultra‐high photogain (η) ≈8.8 × 105, fast response time ≈9 ms, and specific detectivity (D*) ≈8.2 × 1012 J. These unique features are much higher than those of recently published phototransistors configured with other few‐layered 2D single crystals, making ultrathin SnSSe a highly qualified candidate for next‐generation optoelectronic applications. Ultra‐thin layered ternary single crystals of Sn(SxSe1−x)2 with bandgap engineering are proposed to serve as phototransistors, capable of conducting photodetection with high photoresponsivities up to 6000 A W−1. The flexibility and excellent performance of the phototransistor on polyethylene terephthalate substrates make it a promising candidate for next‐generation opto‐electronic applications.