The light curve of KIC 8462852, aka Boyajian's Star, undergoes deep dips the origin of which remains unclear. A faint star ≈2'' to the east was discovered in Keck/NIRC2 imaging in Boyajian et al., but its status as a binary, and possible contribution to the observed variability, was unclear. Here, we use three epochs of Keck/NIRC2 imaging, spanning 5 yr, in JHK near-infrared bands to obtain 1 mas precision astrometry. We show that the two objects exhibit common proper motion, measure a relative velocity of μ = 0.14 ± 0.44 mas yr−1 (μ = 0.30 ± 0.93 km s−1) and conclude that they are a binary pair at 880 ± 10 au projected separation. There is marginal detection of possible orbital motion, but our astrometry is insufficient to characterize the orbit. We show that two other point sources are not associated with KIC 8462852. We recommend that attempts to model KIC 8462852 A's light curve should revisit the possibility that the bound stellar companion may play a role in causing the irregular brightness variations, for example, through disruption of the orbits of bodies around the primary due to long-term orbital evolution of the binary orbit.