Two-dimensional (2D) high-entropy transition metal dichalcogenides (HETMDs) have gained significant interest due to their structural properties and correlated possibilities for high-end devices. However, the controlled synthesis of 2D HETMDs presents substantial challenges owing to the distinction in the inherent characteristics among diverse metal elements in the synthesis, such as saturated vapor pressure of precursors and formation energy of products. Here, we present the synthesis of a 2D HETMD single crystal with 0.92 nm thickness through a liquid-phase reaction system, where the metal elements are fed uniformly and simultaneously. The rapid codeposition of different precursors facilitates the formation of high-entropy products, thereby preventing phase separation. The method can be expanded to produce a variety of 2D HETMDs, such as quinary (MoNbTaV)S2, hexahydroxy (MoWNbTaV)S2, and multichalcogenide (MoWNb)SSe. The as-prepared 2D HETMD is an excellent catalyst for the hydrogen evolution reaction (HER), demonstrating the overpotential of 84 mV at 10 mA cm-2 of an individual crystal, which is much better than that of pristine MoS2 (260 mV at 10 mA cm-2). The strategy offers the flexibility to artificially design the element selectivity and properties of HETMD single crystals in the 2D limit, enabling applications across a wide range of advanced fields.