We report THEMIS observations of a dipolarization front, a sharp, large-amplitude increase in the Z-component of the magnetic field. The front was detected in the central plasma sheet sequentially at X = -20.1 R E (THEMIS P1 probe), at X = -16.7 R E (P2), and at X = -11.0 R E (P3/P4 pair), suggesting its earthward propagation as a coherent structure over a distance more than 10 R E at a velocity of 300 km/s. The front thickness was found to be as small as the ion inertial length. Comparison with simulations allows us to interpret the front as the leading edge of a plasma fast flow formed by a burst of magnetic reconnection in the midtail.