  Here we report time-resolved broadband transient reflectivity measurements
performed in a single crystal of SnS$_2$. We made use of time-domain Brillouin
scattering and a broadband probe to measure the out-of-plane longitudinal sound
velocity $v_L$ = (2950 $\pm$ 100) m s$^{-1}$, in this semiconducting
two-dimensional transition metal dichalcogenide. Our study illustrates the
potential of this non-invasive all-optical pump-probe technique for the study
of the elastic properties of transparent brittle materials and provides the
value of the elastic constant $c_{33}$ = (39 $\pm$ 3) GPa.
