In this work, we present a thermally-controlled coiled polymeric wire as a novel variable stiffness element for robotics applications. We characterized the stiffness behavior of a single twisted nylon wire with 1 mm diameter at different operating temperatures between 25 °C to 50 °C. We observed a linear relationship between the temperature and the stiffness of the nylon wire. This is promising since it would facilitate straightforward adjustment of spring behavior in variable stiffness actuated systems. In order to see the stiffness modulation through temperature change, we conducted dynamic tests on a single joint mechanism with two nylon springs connected in an antagonistic configuration. Specifically, the nylon springs were immersed in water and the temperature was controlled using a Peltier cell. Experimental results indicate that the natural frequency and the damping ratio of the second order system are proportional and inversely proportional to the temperature, respectively.