The thermal conductivity of individual bismuth nanowires was characterized using a suspended microdevice and correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk bismuth in the same crystal direction perpendicular to the trigonal axis, the thermal conductivity of a single-crystal bismuth nanowire of 232 nm diameter was found to be three to six times smaller than bulk in the temperature range between 100 and 300 K, and those of polycrystalline bismuth nanowires of 74–255 nm diameter are reduced by factors of 18–78 over the same temperature range. The thermal conductivity suppression in the single-crystal nanowire can be explained by a transport model that considers diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 10¹⁹ cm ^-3 . The comparable thermal conductivity values measured for polycrystalline nanowires of different diameters suggests a grain boundary scattering mean free path for all heat carriers in the range of 15–40 nm, which is smaller than the nanowire diameters.