  High mass X-ray binaries hosting red supergiant (RSG) donors are a rare but
crucial phase in massive stellar evolution, with only one source previously
known in the Milky Way. In this letter, we present the identification of the
second Galactic RSG X-ray binary SWIFT J0850.8-4219. We identify the source
2MASS 08504008-4211514 as the likely infrared counterpart with a chance
coincidence probability $\approx 5 \times 10^{-6}$. We present a $1.0 -
2.5\,\mu$m spectrum of the counterpart, exhibiting features characteristic of
late-type stars and an exceptionally strong He I emission line, corroborating
the identification. Based on i) the strength of the $^{12}$CO(2,0) band, ii)
strong CN bandheads and absent TiO bandheads at $\approx 1.1\,\mu$m and iii)
equivalent width of the Mg I $1.71\,\mu$m line, we classify the counterpart to
be a K3$-$K5 type RSG with an effective temperature of $3820 \pm 100$ K,
located at a distance of $\approx 12$ kpc. We estimate the source X-ray
luminosity to be $(4 \pm 1) \times 10^{35}$ erg s$^{-1}$, with a hard photon
index ($\Gamma < 1$), arguing against a white dwarf accretor but consistent
with a magnetized neutron star in the propeller phase. Our results highlight
the potential of systematic NIR spectroscopy of Galactic hard X-ray sources in
completing our census of the local X-ray binary population.
