In an effort to find new thermoelectric materials, a series of half-Heusler alloys Sc1-xTmxNiSb (x = 0.0, 0.25, 0.5, 0.75, 1.0) was synthesized by arc melting and subsequent high-pressure high-temperature sintering. The products were examined by means of X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectrometry. The crystal structure of each sample was established to be of the MgAgAs-type, and the cubic lattice parameter was found to behave in accordance to the Vegard's law. The electrical transport properties of the Sc1-xTmxNiSb alloys were determined via resistivity and thermoelectric power measurements performed at temperatures 2-950 K. For each sample a semiconducting-like behavior was found with rather small values of the room-temperature resistivity (rho = 10-50 mu Omega m), and fairly large positive Seebeck coefficient (S-max = 80-136 mu V/K). For the alloy Sc0.75Tm0.25NiSb, combination of relatively small rho (similar to that of TmNiSb) with rather high S (comparable to that of ScNiSb) yielded the thermoelectric power factor PF = 1.2 x 10 mW/(m K-2) at 650 K, which is about twice larger than that found for ScNiSb. Increasing the amount of Tm in Sc1-xTmxNiSb brings about significant reduction in the thermal conductivity at 350 K from about 6 W/(m K) to ca. 3 W/(m K), and consequently leads to sizable enhancement of the thermoelectric figure of merit.