Loop-powered control of pneumatic process control devices is disclosed. A disclosed example apparatus includes an interface for use with a pneumatic process control device of a process control system. The interface includes a power input to scavenge power from a loop power control signal associated with the process control system, a sensor to determine a position of a movable control input associated with the process control device, a comparator to compare the determined position of the movable control input with a desired position of the movable control input, and an actuator to cause movement of the movable control input based on the comparison of the determined position with the desired position, where the actuator is to be powered with the scavenged power.