We have characterized the effective linear electro-optic coefficients of YCa4O(BO3)3 (YCOB) relative to KH2PO4 and KD2PO4 at 632.8 nm. We measured a maximum r(eff) value of 10.8 +/- 1.4 pm/V for YCOB in a transverse electric field configuration for propagation along the X or the alpha dielectric axis, with the electric field applied along the Z or the gamma dielectric axis. We also found effective coefficients of 10.7 +/- 1.0 and 3.4 +/- 0.4 pm/V for YCOB in longitudinal configurations. The remaining values of r(eff) for various transverse applied voltages were found to be less than 3 pm/V. The excellent thermomechanical properties of this crystal, coupled with moderate electro-optic coefficients, make YCOB and its isomorphs potential candidates for use as high-average-power electro-optic switches.