Bi1−xSbx solid solutions have attracted much attention as promising thermoelectric (TE) materials for cooling devices at temperatures below ∼200 K and as unique model materials for solid-state science because of a high sensitivity of their band structure to changes in composition, temperature, pressure, etc. Earlier, we revealed a non-monotonic behavior of the concentration dependences of TE properties for polycrystalline Bi1−xSbx solid solutions and attributed these anomalies to percolation effects in the solid solution, transition to a gapless state, and to a semimetal–semiconductor transition. The goal of the present work is to find out whether the non-monotonic behavior of the concentration dependences of TE properties is observed in the thin film state as well. The objects of the study are Bi1−xSbx thin films with thicknesses in the range d = 250–300 nm prepared by thermal evaporation of Bi1−xSbx crystals (x = 0–0.09) onto mica substrates. It was shown that the anomalies in the dependence of the TE properties on Bi1−xSbx crystal composition are reproduced in thin films.