During the last two decades, PT-based relaxor ferroelectric crystals such as (1−x)Pb(Zn 1/3 Nb 2/3 )O 3 -xPbTiO 3 (PZN-PT), (1−x)Pb(Mg 1/3 Nb 2/3 )O 3 -xPbTiO 3 (PMN-PT), and (1−x−y)Pb(In 1/2 Nb 1/2 )O 3 -yPb(Mg 1/3 Nb 2/3 )O 3 -xPbTiO 3 (PIN-PMN-PT or PIMNT) crystals, are widely studied due to their huge piezoelectric properties and super-high electro-mechanical coupling factors. They have excellent properties with compositions around the morphotropic phase boundary (MPB). It is gratifying that the PT-based relaxor crystals are replacing traditional PZT piezo-ceramics in many application fields. Most ferroelectrics with oxygen-octahedral structure, which show outstanding electro-mechanical properties, also have excellent optical performances. Ferroelectric single crystals with large electro-optic (EO) modulation are widely applied in laser communication devices. For the practical applications, the knowledge of detailed optical parameters is desirable. This paper tries to show a global review on the optical properties of PT-based relaxor ferroelectric crystals. In the present review, optical properties of the crystals are systematically summarized, including refractive index dispersion, transmittance, band gap, EO, acousto-optic, and photorefractive properties. These properties change with the crystal composition, orientation, and poling condition. The purpose of the review is to provide a resource for the researchers who are concerned with basic physical investigation or optical device applications of the PT-based relaxor ferroelectric crystals.