We present deep charge coupled device (CCD) imaging of the low-luminosity radio galaxy B2 1243+26 which contains a two-sided, well-defined radio jet at the kpc scale. The galaxy is located in a region of high galactic density and has a nearby companion of similar luminosity. At approximately 10 kpc from the nucleus of the galaxy, we detect optical emission spatially associated with the radio jet. This structure is elongated and extends parallel to it by approximately 24 kpc. We discuss the nature of the optical emission and its relation to the radio jet. Photoionization of gas by a nuclear source or by shocks, similar to that occurring in other radio galaxies, cannot be ruled out. On the other hand, our observations are consistent with optical emission from the ambient gas that has been emtrained by the radio jet. This hypothesis will explain nicely the high asymmetry in the polarization of the two sides of the source, which is otherwise very symmetric.