A search for Lorentz- and CPT-violating signals in the double beta decay spectrum of 136-Xe has been performed using an exposure of 100 kg yr with the EXO-200 detector. No significant evidence of the spectral modification due to isotropic Lorentz-violation was found, and a two-sided limit of -2.65 x 10^5 GeV < a^(3)_of < 7.60 x 10^-6 GeV (90% C.L.) is placed on the relevant coefficient within the Standard-Model Extension (SME). This is the first experimental study of the effect of the SME-defined oscillation-free and momentum-independent neutrino coupling operator on the double beta decay process.