We have measured astrometry for members of the Orion Nebula Cluster with images obtained in 2015 with the Wide Field Camera 3 on board the Hubble Space Telescope. By comparing those data to previous measurements with the Near-Infrared Camera and Multi-Object Spectrometer on Hubble in 1998, we have discovered that a star in the Kleinmann-Low Nebula, source x from Lonsdale et al., is moving with an unusually high proper motion of 29 mas yr super(?1), which corresponds to 55 km s super(?1) at the distance of Orion. Previous radio observations have found that three other stars in the Kleinmann-Low Nebula (the Becklin-Neugebauer object and sources I and n) have high proper motions (5-14 mas yr super(?1)) and were near a single location ~540 years ago, and thus may have been members of a multiple system that dynamically decayed. The proper motion of source x is consistent with ejection from that same location 540 years ago, which provides strong evidence that the dynamical decay did occur and that the runaway star BN originated in the Kleinmann-Low Nebula rather than the nearby Trapezium cluster. However, our constraint on the motion of source n is significantly smaller than the most recent radio measurement, which indicates that it did not participate in the event that ejected the other three stars.