  We present the discovery of a variable optical counterpart to the
unidentified gamma-ray source 3FGL J0212.1+5320, and argue this is a new
compact binary millisecond pulsar (MSP) candidate. We show 3FGL J0212.1+5320
hosts a semi-detached binary with a 0.86955$\pm$0.00015 d orbital period and a
F6-type companion star at an estimated distance of D=1.1$\pm$0.2 kpc, with a
radial velocity curve semi-amplitude K$_2$=214.1$\pm$5.0 km s$^{-1}$ and a
projected rotational velocity of Vsin(i)=73.2$\pm$1.6 km s$^{-1}$. We find a
hard X-ray source at the same location with a 0.5$-$10 keV luminosity
L$_\mathrm{X}$=2.6$\times$10$^{32}$ (D/1.1 kpc)$^2$ erg s$^{-1}$, which
strengthens the MSP identification. Our results imply a mass ratio
q=M$_2$/M$_1$=0.26$^{+0.02}_{-0.03}$ if the companion star fills its Roche
lobe, and q$\gtrsim$0.26 in any case. This classifies 3FGL J0212.1+5320 as a
"redback" binary MSP; if its MSP nature is confirmed, this will be the
brightest compact binary MSP in the optical band (r'$\simeq$14.3 mag) and will
have the longest orbital period among Galactic field systems (nearly 21 hr).
Based on the light curve peak-to-peak amplitude ($\Delta$r=0.19 mag), we
further suggest that the orbital inclination is high and the putative pulsar
mass is close to canonical (M$_1$$\simeq$1.3$-$1.6 M$_\odot$). Finally, we
discuss the lack of heating signatures and asymmetric optical light curves in
the context of other redback MSPs.
