We shall present the discovery of the first extragalactic Zeeman-splitting detections seen in emission from OH megamasers. We conducted a survey of OH megamaser emission from 6 ULIRGs (including the archetypal ULIRG Arp 220) using the Arecibo 300-m telescope and measured the Zeeman effect in 16 masing regions in 4 of the 6 galaxies. These measurements suggest a typical field of about 3 mG in ULIRGs. Field reversals were seen in 3 of the galaxies and an amazing 7 Zeeman-splitting detections were found in Arp 220 alone. Our largest detected field, with a line-of-sight magnitude of 18 mG, was seen in one of only three known OH gigamasers, at a redshift of 0.217. Prior to these observations, Zeeman splitting had only been seen in one extragalactic source and, at that, in absorption of HI against a background source. Our results demonstrate that OH megamasers are a sensitive extragalactic magnetometer and a promising new tool for probing the astrophysics of distant luminous star-forming galaxies.