Interfacing nanomechanics with photonics and charge/spin-based electronics has transformed information technology and facilitated fundamental searches for the quantum-to-classical transition1–3. Utilizing the electron valley degree of freedom as an information carrier, valleytronics has recently emerged as a promising platform for developments in computation and communication4–7. Thus far, explorations of valleytronics have focused on optoelectronic and magnetic means8–16. Here, we realize valley–mechanical coupling in a resonator made of the monolayer semiconductor MoS2 and transduce valley information into mechanical states. The coupling is achieved by exploiting the magnetic moment of valley carriers with a magnetic field gradient. We optically populate the valleys and observe the resulting mechanical actuation using laser interferometry. We are thus able to control the valley–mechanical interaction by adjusting the pump-laser light, the magnetic field gradient and temperature. Our work paves the way for realizing valley-actuated devices and hybrid valley quantum systems.