The linear electrooptic coefficients of Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) single crystals in the rhombohedral and tetragonal phases were characterized at a wavelength of 632.8 nm. Large electrooptic responses were observed. The tetragonal 0.62PMN–0.38PT single crystals poled along the <001> direction (the spontaneous polarization direction) show r33=70 pm/V, r13=25 pm/V, and r51=558 pm/V. In addition, r33 and r13 of crystals of this composition show a slight increase with temperature, which originates from the temperature dependence of polarization and dielectric constant. The rhombohedral 0.67PMN–0.33PT single crystals poled along the <111> direction have r33=107 pm/V, r13∼0, and r51=1050 pm/V.