  We present a multi-wavelength study of the unidentified Fermi object, 3FGL
J0212.1+5320. Within the 95% error ellipse, Chandra detects a bright X-ray
source (i.e., F(0.5-7keV) = 1.4e-12 erg/cm^2/s), which has a low-mass optical
counterpart (M < 0.4 Msun and T ~ 6000 K). A clear ellipsoidal modulation is
shown in optical/infrared at 20.87 hours. The gamma-ray properties of 3FGL
J0212.1+5320 are all consistent with that of a millisecond pulsar, suggesting
that it is a gamma-ray redback millisecond pulsar binary with a low-mass
companion filling > 64% of the Roche-lobe. If confirmed, it will be a redback
binary with one of the longest orbital periods known. Spectroscopic data taken
in 2015 from the Lijiang observatory show no evidence of strong emission lines,
revealing that the accretion is currently inactive (the rotation-powered pulsar
state). This is consistent with the low X-ray luminosities (Lx ~ 10^32 erg/s)
and the possible X-ray modulation seen by Chandra and Swift. Considering that
the X-ray luminosity and the high X-ray-to-gamma-ray flux ratio (8%) are both
comparable to that of the two known gamma-ray transitional millisecond pulsars,
we suspect that 3FGL J0212.1+5320 could be a potential target to search for
future transition to the accretion active state.
