Viscoelastic behavior (modulus and damping) of polycrystalline 3%KNbO3-BaTiO3 has been studied in torsion and bending via broadband viscoelastic spectroscopy (BVS) over a range of temperature passing through both the orthorhombic-to-tetragonal and tetragonal-to-cubic phase transformations. Softening in bulk modulus and minima in Poisson's ratio as well as peaks in mechanical damping, dielectric constant, and heat capacity vs. temperature were observed in the vicinity of the transformations. Viscoelastic response vs. temperature was sharper than dielectric or thermal response. Transition temperatures of both transformations shifted to lower temperatures in comparison to pure BaTiO3. Damping peaks vs. temperature were wider (5 °C) than in pure BaTiO3 (less than 1 °C) and of smaller magnitude. Broadening of response is attributed to material heterogeneity on a fine scale.