We study magnetic and magnetotransport properties of an epitaxial interfacial multiferroic system consisting of a ferromagnetic Heusler-alloy Co 2 FeSi and a ferroelectric-oxide BaTiO 3 . L2 1 -ordered Co 2 FeSi epilayers on BaTiO 3 (001) show an in-plane uniaxial magnetic anisotropy with strong temperature dependence, induced by the presence of the magnetoelastic effect via the spin-orbit interaction at the Co 2 FeSi/BaTiO 3 (001) interface. In the Co 2 FeSi Hall-bar devices, the anisotropic magnetoresistance (AMR) hysteretic curves depending on inplane magnetization reversal processes on the a and c domains of BaTiO 3 (001) are clearly observed at room temperature. Notably, the magnitude of the AMR ratio (%) for Co 2 FeSi Hall-bar devices can be tuned through the a − c domain wall motion of BaTiO 3 (001) by applying electric fields. We propose that the tunable AMR effect is associated with the modulation of the spin-orbit interaction, exchange interaction, and/or the electronic band structure near the Fermi level by applying electric fields in the epitaxial Co 2 FeSi/BaTiO 3 (001) interfacial multiferroic system.