Dielectric permittivity, domain structure, and hysteresis loop have been measured as a function of temperature in a (001)-cut Pb(In1/2Nb1/2)0.6Ti0.4O3 (PINT40%) single crystal. A first-order-type tetragonal/monoclinic→cubic phase transition takes place at TC≅486 K, below which tetragonal and monoclinic domains coexist. Frequency-dependent maxima in dielectric absorption appear in the region of 180–260 K and can be described by the Vogel-Fulcher equation. Optical transmission and refractive indices were obtained as a function of wavelength. The crystal shows a broad transparency (0.4–6.0 μm) and no obvious birefringence, indicating that the crystal structure averaged over the optically anisotropic ferroelectric domains is optically isotropic.