The present investigation describes the optical properties of tin sulfide (SnS) thin films that were deposited on fluorine-doped tin oxide (FTO)-coated glass substrates at room temperature using the thermal evaporation method. The obtained films showed oriented growth with “p”-type conductivity. The effect of film thickness on the optical behavior of FTO/SnS was analyzed and compared with data from SnS films grown on glass and indium tin oxide substrates. Our study indicates that the properties of SnS film are independent of the substrate material. The optical band gap was found to decrease from 2.07 to 1.30 eV with increasing film thickness. We have seen a quantum confinement effect in samples whose grain size was less than 27 nm. The refractive indices of the samples were used to determine the single oscillator and dispersion energies using the Wemple–DiDomenico single-oscillator model. Other optical parameters were also determined using the transmission and absorption spectra. Besides grain size and number of defects, our data showed that the polarizability of the molecules along the Van der Waals direction influenced SnS optical properties. The interpretation was made possible considering the carrier concentration of free charges remained constant with varying film thickness. Such work provides insight into how to choose the appropriate thickness and, hence, grain sizes for optoelectronic applications.