  We report the appearance of a new radio source at a projected offset of 460
pc from the nucleus of Cygnus A. The flux density of the source (which we
designate Cygnus A-2) rose from an upper limit of <0.5 mJy in 1989 to 4 mJy in
2016 (nu=8.5 GHz), but is currently not varying by more than a few percent per
year. The radio luminosity of the source is comparable to the most luminous
known supernovae, it is compact in VLBA observations down to a scale of 4 pc,
and it is coincident with a near-infrared point source seen in pre-existing
adaptive optics and HST observations. The most likely interpretation of this
source is that it represents a secondary supermassive black hole in a close
orbit around the Cygnus A primary, although an exotic supernova model cannot be
ruled out. The gravitational influence of a secondary SMBH at this location may
have played an important role in triggering the rapid accretion that has
powered the Cygnus A radio jet over the past 10^7 years.
