Metamaterials exhibit numerous novel effects and operate over a large portion of the electromagnetic spectrum. Metamaterial devices based on these effects include gradient-index lenses, modulators for terahertz radiation and compact waveguides. The resonant nature of metamaterials results in frequency dispersion and narrow bandwidth operation where the centre frequency is fixed by the geometry and dimensions of the elements comprising the metamaterial composite. The creation of frequency-agile metamaterials would extend the spectral range over which devices function and, further, enable the manufacture of new devices such as dynamically tunable notch filters. Here, we demonstrate such frequency-agile metamaterials operating in the far-infrared by incorporating semiconductors in critical regions of metallic split-ring resonators. For this first-generation device, external optical control results in tuning of the metamaterial resonance frequency by ∼20%. Our approach is integrable with current semiconductor technologies and can be implemented in other regions of the electromagnetic spectrum.