We propose an electrostatically actuated spiral structure as a metamaterial for circularly polarized light in the terahertz (THz) frequency range. An array of planar spiral structures was fabricated with micro electro mechanical system technology, and the geometry of the structures can be changed by electrostatic actuation. The actuation transforms the planar spirals into three-dimensional helices, resulting in optical activity in which the differential polarization response of the material depends on whether the incident light is left- or right-circularly polarized. THz spectroscopy confirmed that the optical activity can be tuned with the proposed structures. (C) 2013 AIP Publishing LLC.