  As a natural consequence of cosmological hierarchical structure formation,
sub-parsec supermassive black hole binaries (SMBHBs) should be common in
galaxies but thus far have eluded spectroscopic identification. Based on four
decades of optical spectroscopic monitoring, we report that the nucleus of NGC
5548, a nearby Seyfert galaxy long suspected to have experienced a major merger
about one billion years ago, exhibits long-term variability with a period of 14
years in the optical continuum and broad Hbeta emission line. Remarkably, the
double-peaked profile of Hbeta shows systematic velocity changes with a similar
period. These pieces of observations plausibly indicate that a SMBHB resides in
the center of NGC 5548. The complex, secular variations in the line profiles
can be explained by orbital motion of a binary with equal mass and a semi-major
axis of 22 light-days (corresponding to 18 milli-parsec). At a distance of 75
Mpc, NGC 5548 is one of the nearest sub-parsec SMBHB candidates that offers an
ideal laboratory for gravitational wave detection.
