We observe and systematically tune an intense mid-infrared absorption mode that results from phosphorus doping in silicon nanowires synthesized via the vapor-liquid-solid technique. The angle- and shape-dependence of this spectral feature, as determined via in-situ transmission infrared spectroscopy, supports its assignment as a longitudinal localized surface plasmon resonance (LSPR). Modulation of resonant frequency (740-1620 cm(-1)) is accomplished by varying nanowire length (135-1160 nm). The observed frequency shift is consistent with Mie-Gans theory, which indicates electrically active dopant concentrations between 10(19) and 10(20) cm(-3). Our findings suggest new opportunities to confine light in this ubiquitous semiconductor and engineer the optical properties of nontraditional plasmonic materials.