  We present the results of optical, near-infrared, and mid-infrared
observations of M101 OT2015-1 (PSN J14021678+5426205), a luminous red transient
in the Pinwheel galaxy (M101), spanning a total of 16 years. The lightcurve
showed two distinct peaks with absolute magnitudes $M_r\leq-12.4$ and $M_r
\simeq-12$, on 2014 November 11 and 2015 February 17, respectively. The
spectral energy distributions during the second maximum show a cool outburst
temperature of $\approx$3700 K and low expansion velocities ($\approx-$300
\kms) for the H I, Ca II, Ba II and K I lines. From archival data spanning 15
to 8 years before the outburst, we find a single source consistent with the
optically discovered transient which we attribute to being the progenitor; it
has properties consistent with being an F-type yellow supergiant with
$L$~$\sim$~8.7~$\times\ 10^4$ \Lsun, $T_{\rm{eff}}\approx$7000~K and an
estimated mass of $\rm{M1}= 18\pm 1$ \Msun. This star has likely just finished
the H burning phase in the core, started expanding, and is now crossing the
Hertzsprung gap. Based on the combination of observed properties, we argue that
the progenitor is a binary system, with the more evolved system overfilling the
Roche lobe. Comparison with binary evolution models suggests that the outburst
was an extremely rare phenomenon, likely associated with the ejection of the
common envelope. The initial mass of the binary progenitor system fills the gap
between the merger candidates V838 Mon (5$-$10 \Msun) and
NGC~4490-OT~(30~\Msun).
