Advanced microelectronics in the future may require semiconducting channel materials beyond silicon. Two-dimensional (2D) semiconductors, with their atomically thin thickness, hold great promise for future electronic devices. One challenge to achieving high-performance 2D semiconductor field effect transistors (FET) is the high contact resistance at the metal-semiconductor interface. In this study, we develop a charge-transfer doping strategy with WSe₂/α-RuCl₃ heterostructures to achieve low-resistance ohmic contact for p-type monolayer WSe₂ transistors. We show that hole doping as high as 3 × 10¹³ cm^-2 can be achieved in the WSe₂/α-RuCl₃ heterostructure due to its type-III band alignment, resulting in an ohmic contact with resistance of 4 kΩ μm. Based on that, we demonstrate p-type WSe₂ transistors with an on-current of 35 μA·μm^-1 and an I_ON/I_OFF ratio exceeding 10⁹ at room temperature.