Thermoelectric transport properties of InGaAs with a high concentration of embedded semimetallic TbAs nanoparticles were studied in a 300–600 K temperature range. The TbAs concentrations studied were over 1% in a regime where coherent multiple scattering could affect electron transport. Scattering by ionized embedded nanoparticles, which are approximately one nanometer in diameter, is significantly different from scattering by shallow ionized impurities. The Seebeck coefficient and electrical conductivity were characterized as a function of temperature. The coefficients showed unusual behavior for a typical semiconductor, both growing with temperature. However, drastic worsening of the electrical conductivity compared with Si-doped InGaAs was also observed. The measured room temperature thermal conductivities of TbAs:InGaAs nanocomposites were lower than those of Si-doped InGaAs. Nonetheless, the thermoelectric performance was still governed by the electrical conductivity, and no enhancement of the thermoelectric figure of merit ZT was achieved at room temperature.