A longitudinal electro‐optic memory effect is observed in ferroelectric single‐crystal SrxBa1−xNb2O6with tetragonal symmetry. The changeable birefringence depends on the remanent polarization of the crystal and can be switched by voltage pulses. These optical characteristics, which seem to be similar to the optical properties observed in ferroelectric Pb3MgNb2O9with a diffuse phase transition, can be utilized in light‐valve devices which operate in the longitudinal mode without a transverse electric field or strain bias.