AbstractWe report a comprehensive study of the nanoscale inhomogeneity and disorder on the thermoelectric properties of FeSe$_{1-x}$ 1 - x S$_x$ x ($0 \le x \le 1$ 0 ≤ x ≤ 1 ) single crystals and the evolution of correlation strength with S substitution. A hump-like feature in temperature-dependent thermpower is enhanced for x = 0.12 and 0.14 in the nematic region with increasing in orbital-selective electronic correlations, which is strongly suppressed across the nematic critical point and for higher S content. Nanoscale Se/S atom disorder in the tetrahedral surroundings of Fe atoms is confirmed by scanning transmission electron microscopy measurements, providing an insight into the nanostructural details and the evolution of correlation strength in FeSe$_{1-x}$ 1 - x S$_x$ x .