Deep vein thrombosis (DVT) is a severe medical condition that affects many patients around the world, where one of the main causes is commonly associated with prolonged immobilization. Current mechanical prophylaxis systems, such as the compression stockings and intermittent pneumatic compression devices, have yet to show strong efficacy in preventing DVT. The current study aimed to develop a soft pneumatic sock prototype that uses soft extension pneumatic actuators to provide assisted ankle dorsiflexion–plantarflexion motion, so as to prevent the occurrence of DVT. The prototype was evaluated for its efficacy to provide the required dorsiflexion–plantarflexion motion by donning and actuating the prototype on simulated ankle–foot models with various ankle joint stiffness values. Our results showed that the soft extension actuators in the sock prototype provided controllable assisted ankle plantarflexion through actuator extension and ankle dorsiflexion through actuator contraction, where in our study, the actuations extended to 129.9–146.8% of its original length. Furthermore, the sock was able to achieve consistent range of motion at the simulated ankle joint across different joint stiffness values (range of motion: 27.5 ± 6.0 deg). This study demonstrated the feasibility of using soft extension pneumatic actuators to provide robot-assisted ankle dorsiflexion–plantarflexion motion, which will act as an adjunct to physiotherapists to optimize therapy time for bedridden patients and therefore may reduce the risk of developing DVT.