Here we demonstrate a new class of designer plasmonic materials for use in the mid-infrared (mid-IR) region of the electromagnetic spectrum. By heavily doping epitaxially-grown semiconductor materials, we are able to grow single-crystal materials whose optical properties in the mid-IR mimic those of metals at shorter wavelengths. We demonstrate materials with plasma frequencies from 5.5 × 15μm and low losses, compared to their shortwavelength counterparts. In addition, we demonstrate the ability of subwavelength particles formed from our materials to support localized surface plasmon resonances, and measure the near-field absorption of these structures using a novel nanoscale infrared spectroscopy technique. Finally, we show good agreement between our observed results and analytical and finite-element models of our materials and structures. The results presented offer a path towards nanoscale confinement of light with micron-scale wavelengths.