  Swift J0243.6+6124 was discovered during a giant X-ray outburst in October
2017. While there are numerous studies in the X-ray band, very little is known
about the optical counterpart. We have performed an spectral and photometric
analysis of the optical counterpart of this intriguing source. We find that the
optical counterpart to Swift J0243.6+6124 is a V = 12.9, O9.5Ve star, located
at a distance of $\sim5$ kpc. The optical extinction in the direction of the
source is $A_V=3.6$ mag. The rotational velocity of the O-type star is 210 km
s$^{-1}$. The long-term optical variability agrees with the growth and
subsequent dissipation of the Be circumstellar disk after the giant X-ray
outburst. The optical and X-ray luminosity are strongly correlated during the
outburst, suggesting a common origin. We did not detect short-term periodic
variability that could be associated with nonradial pulsations from the Be star
photosphere.
