Elastic anisotropy and slowness curves of barium titanate (BaTiO3) single crystal in the paraelectric phase were investigated by using Brillouin spectroscopy. The directional dependence of sound velocities of the longitudinal and the transverse acoustic waves in the (001) plane showed a four-fold symmetry typical in the cubic phase. Especially, one of the two transverse modes exhibited a substantial softening along the <110> directions. The sound velocity of this mode decreased significantly as temperature decreased toward the transition temperature, which corresponded to the elastic stiffness coefficient (C11 – C12)/2. This result suggests the tetragonal instability caused by the precursor polar clusters in the paraelectric phase of BaTiO3.