Fabricating responsive hydrogels via additive manufacturing is a growing research area. Hydrogel actuators generate mechanical motion due to external stimulus in a controlled manner. This paper presents an investigation on the current 3D printed hydrogel actuators including their fabrication process, and their applications spanning from soft robots to medical devices. A discussion of the reviewed 3D printed hydrogel soft actuators is given focusing on external stimulus and control. The finding of this study is that the integration of the 3D printing technology and hydrogel materials enables new possibilities in the production of customized soft actuators for a range of real-wold applications.