AbstractPhase engineering of nanomaterials (PEN) has demonstrated great potential in the fields of catalysis, electronics, energy storage and conversion, and condensed matter physics. Recently, transition metal dichalcogenides (TMDs) with unconventional metastable phases (e.g., 1T and 1T′) have attracted increasing research interest due to their unique and appealing physicochemical properties. However, there is still a lack of a simple, universal, and controlled method for the preparation of large‐scale and high‐purity unconventional‐phase TMD crystals, restricting their further fundamental study and practical applications. Here, a facile, one‐step salt‐assisted general strategy is reported for the controlled phase transformation of commercially available TMDs with conventional 2H phase, yielding a large amount of metastable 1T′‐phase TMDs, including WS₂, WSe₂, MoS₂, and MoSe₂. It is found that the easily accessible metal salts, such as K₂C₂O₄·H₂O, K₂CO₃, Na₂CO₃, Rb₂CO₃, Cs₂CO₃, KHCO₃, NaHCO₃, and NaC₂O₄, can be used to assist the 2H‐to‐1T′ phase transformation, greatly simplifying the synthetic process for producing metastable 1T′‐TMDs. Importantly, this method can also be used to prepare 1T′‐TMD alloys, such as 1T′‐WS_2xSe_2(1−x). This newly developed strategy is robust and highly effective, which can also be used for the phase engineering of other materials with various polymorphs.