3C 20 is a radio galaxy located in the constellation Cassiopeia. It is one of largest known galaxies with a diameter of 850,000 ly (260 kpc). The galaxy features a prominent double hotspot in its eastern lobe. An unusually low fraction of the flux lies within the radio core, which is suggested to be caused by a combination of factors, including jet orientation, synchrotron self-absorption and aging, as well as interactions with surrounding gas and dust.