We present reversible and a-priori control of the polarization of a photon emitted by a single molecule by introducing a nanoscale metal object in its near field. It is experimentally shown that, with the metal close to the emitter, the polarization ratio of the emission can be varied by a factor of 2. The tunability of polarization decays, when the metal is displaced by typically 30 nm. Calculations based on the multiple multipole method agree well with our experiments and predict even further enhancement with a suitable nanoantenna design.