  In Hubble Space Telescope (HST) imaging taken on 10 November 2014, four
images of supernova (SN) "Refsdal" (redshift z=1.49) appeared in an
Einstein-cross--like configuration (images S1-S4) around an early-type galaxy
in the cluster MACS J1149.5+2223 (z=0.54). Almost all lens models of the
cluster have predicted that the SN should reappear within a year in a second
host-galaxy image created by the cluster's potential. In HST observations taken
on 11 December 2015, we find a new source at the predicted position of the new
image of SN Refsdal approximately 8" from the previous images S1-S4. This marks
the first time the appearance of a SN at a particular time and location in the
sky was successfully predicted in advance! We use these data and the light
curve from the first four observed images of SN Refsdal to place constraints on
the relative time delay and magnification of the new image (SX), compared to
images S1-S4. This enables us, for the first time, to test "blind" lens model
predictions of both magnifications and time delays for a lensed SN. We find
that the timing and brightness of the new image are consistent with the blind
predictions of a fraction of the models. The reappearance illustrates the
discriminatory power of this blind test and its utility to uncover sources of
systematic uncertainty. From planned HST photometry, we expect to reach a
precision of 1-2% on the time delay between S1-S4 and SX.
