The concept, design, and testing of an electric thruster for underwater propulsion based on the electro-osmotic principle are presented. A unique feature of the proposed electro-osmotic thruster (EOT) is the absence of dynamic components, enabling robust, stealthy operation, and the potential for application in extreme underwater conditions. Furthermore, the EOT is unaffected by magnetic fields as it does not require metallic components. In a particularly extreme environment test, a small EOT was immersed in an ultrasonic bath and demonstrated normal operation. In another test, multiple EOTs were aligned to work in parallel to increase packing efficiency. In a large-scale test the EOT successfully propelled a small 5 kg unmanned underwater vehicle (UUV) at 2.2 cm/s. This experiment was performed using a fraction of the EOT's maximum potential thrust that could be available if it were to be mounted to a large-scale autonomous underwater vehicle (AUV) platform, such as a REMUS 100. The EOT described in this paper is the first underwater thruster to continue the abandoned work of magneto-hydrodynamics in finding low-wake steady-state propulsion.