The structural, morphological, optical, and nonlinear optical properties of a lead sulfide (PbS) thin film grown by chemical bath deposition (CBD) are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), ultraviolet-visible (UV-Vis), and open aperture Z-scan experiments. The band gap energy of the PbS nanocrystalline film is 1.82 eV, higher than that of bulk PbS at 300 K. The nonlinear absorption properties of the film are investigated using the open aperture Z-scan technique at 1064 nm and pulse durations of 4 ns and 65 ps. Intensity-dependent switching of the film from nonlinear absorption to saturable absorption is observed. The nonlinear absorption coefficient increases monotonically with increasing pulse duration from 65 ps to 4 ns.