Achieving on-demand control of the valley degree of freedom is essential for valley-based information science and technology. Optical and magnetic control of the valley degree of freedom in monolayer transition-metal dichalcogenide (TMD) semiconductors has been studied extensively. However, electrical control of the valley polarization has remained a challenge. Here we demonstrate switching of the valley polarization in monolayer WSe2 by electrical gating. This is achieved by coupling a WSe2 monolayer to a two-dimensional (2D) layered magnetic insulator CrI3. The valley degeneracy in WSe2 is lifted by the magnetic proximity effect. The valley polarization is switched through gate control of the interlayer spin-flip transition in 2D CrI3, which switches the magnetization of the CrI3 layer adjacent to the WSe2 layer. The effect is manifested by a sign change in the photoluminescence handedness of WSe2. Our results provide the basis for high-speed and energy-efficient gate control of the valley degree of freedom in monolayer TMD semiconductors. © 2020 American Physical Society.