We report the detection of a sudden spin-up of the 11 s anomalous X-ray pulsar 1RXS J170849.0-4000910 in regular timing observations made with the Rossi X-ray Timing Explorer. The event, which occurred between MJD 51446 (1999 September 25) and 51472 (1999 October 21), is well characterized by an increase in the rotational frequency of magnitude (Delta nu / nu) = (6.2 +/- 0.3) x 10^-7 and an increase in the rate of spin down (Delta \dotν/ \dotν) = (1.38 +/- 0.25) x 10^-2. These values are very similar to those of glitches observed in the Vela radio pulsar and other young radio pulsars. The event therefore suggests that the internal structure of this anomalous X-ray pulsar is similar to those of the radio pulsars. In particular, it implies that the fractional moment of inertia in neutron superfluid that is not corotating with the crust is >= 1 %. The detection of a glitch in this anomalous X-ray pulsar constrains models for the origin of glitches in neutron stars. Most notably, it challenges models that preclude glitches in long-period pulsars, and, under the magnetar hypothesis, suggests that large glitches can occur in hot neutron stars. The glitch is consistent with the predictions of the magnetar model for anomalous X-ray pulsars, but accretion-powered scenarios cannot be excluded using our observations alone.

12 pages, 1 figure, accepted for publication in ApJL