A high-voltage metal-oxide-semiconductor (HV MOS) transistor device and a manufacturing method thereof are provided. The HV MOS transistor device includes a semiconductor substrate, a gate structure, a first sub-gate structure, and a drain region. The gate structure is disposed on the semiconductor substrate. The semiconductor substrate has a first region and a second region respectively disposed on two opposite sides of the gate structure. The first sub-gate structure is disposed on the semiconductor substrate, the first sub-gate structure is separated from the gate structure, and the first sub-gate structure is disposed on the first region of the semiconductor substrate. The drain region is disposed in the first region of the semiconductor substrate. The drain region is electrically connected to the first sub-gate structure via a first contact structure disposed on the drain region and the first sub-gate structure.