Current soft actuators rely on additional hardware such as pumps, high voltage supplies, light generation sources and magnetic field generators for their operation. These components resist miniaturization and embedding them into small-scale soft robots would be challenging. This limits their mobile applications where the entire system needs to be untethered especially in hyper-redundant robots where a high number of actuators are needed. Here, we introduce miniature and untethered robots made of soft voxel actuators (SVAs) - an active voxel using stimuli-responsive hydrogels actuated by electrical currents through Joule heating. SVAs weighing only 100 mg require small footprint microcontrollers for their operation which can be embedded in the robotic system. We have demonstrated the advantages of hydrogel-based SVAs through a hyper-redundant manipulator with 16 actuators and an untethered miniature robot for underwater mobile applications.