We report the discovery of Faraday rotation toward radio source 3C 84 , the active galactic nucleus in NGC1275 at the core of the Perseus Cluster . The rotation measure ( RM ) , determined from polarization observations at wavelengths of 1.3 and 0.9 mm , is ( 8.7 \pm 2.3 ) \times 10 ^ { 5 } rad m ^ { -2 } , among the largest ever measured . The RM remained relatively constant over a 2 year period even as the intrinsic polarization position angle wrapped through a span of 300 degrees . The Faraday rotation is likely to originate either in the boundary layer of the radio jet from the nucleus , or in the accretion flow onto the central black hole . The accretion flow probably is disk-like rather than spherical on scales of less than a parsec , otherwise the RM would be even larger .