New bulk nanocomposite thermoelectric materials composed from nanocrystallites of Bi–Sb–Te alloys covered by C60 molecules have been synthesized and studied. The fullerene molecules provide thermal phonons blocking and particular charge transfer in the nanocomposite. The molecules act as electron traps, and thus decrease the density of free electrons in n-type semiconductor and generate holes in p-type materials. The capture of electrons is not accompanied by shifts of the Raman bands in C60 spectra. The density of free charge carriers and their Hall mobility change nonmonotonically with the increase of fullerene content in Bi–Sb–Te–C60 nanocomposites. The maximum value of thermoelectric figure of merit ZT = 1.15 was obtained in p-type nanocomposite that is 30% higher than in the undoped starting material. The observed phenomenon provides new ways for transport properties optimization of thermoelectric materials and thermoelectric devices efficiency increase.