Every crystal surface breaks inversion symmetry even when the bulk is centrosymmetric, creating an intrinsic boundary polarity (P). Here we show that this polarity enables zero-field nonreciprocal transport in the centrosymmetric kagome magnets TbMn₆Sn₆₋ₓInₓ. In symmetric TbMn₆Sn_(4.4)In_(1.6) microdevices, opposing side surfaces exhibit second-harmonic longitudinal resistances of comparable magnitude and opposite sign. Reversing the spontaneous magnetization (M) reverses both signals, whereas rotating M so that the toroidal moment P × M has no component along the current suppresses the response. In pristine TbMn₆Sn₆, the signal switches off across the spin-reorientation transition, providing a chemically ordered control of the same symmetry relation. Spatial potential mapping indicates that the nonlinear boundary contribution is distinct from the bulk linear conduction. Spin-orbit-coupled surface states associated with topological bands may enhance the effect. Crystal-surface polarity therefore offers a broadly applicable route to zero-field rectification in centrosymmetric magnetic conductors around room temperature.