An example robot actuator utilizing a differential pulley transmission is provided. As an example, a differential pulley actuator includes input drive gears for coupling to a motor and timing pulleys coupled together through the input drive gears. Rotation of the input drive gears causes rotation of a first timing pulley in a first direction and rotation of a second timing pulley in a second direction opposite the first direction. The actuator also includes multiple idler pulleys suspended between the timing pulleys and the output pulley, and the multiple idler pulleys are held in tension between the timing pulleys via a first tension-bearing element and the output pulley via a second tension-bearing element. The first tension-bearing element loops around the timing pulleys and the multiple idler pulleys. The output pulley couple to a load, and is configured to apply motion of the multiple idler pulleys to the load.