Phase-change materials are chalcogenide alloys used for nonvolatile memory applications due to their rapid and reversible structural transformation. In3SbTe2 is a promising candidate that exhibits transitions dependent on thermal conductivity. The minimum lattice thermal conductivity of amorphous In3SbTe2 is investigated by surface acoustic propagation. In3SbTe2 thin films were deposited by radio frequency magnetron sputtering on (100) Si. Rutherford backscattering spectrometry and x-ray reflectivity were used to establish the elemental composition, deposition rate, and mass density. Using the Debye model, the thermal conductivity is extracted from fitted phase velocities measured by surface Brillouin scattering. The low thermal conductivity is revealed to be suitable for Joule heating.