B. E. Cobb, J. S. Bloom, A. N. Morgan, S. B. Cenko and D. A. Perley, University of California, Berkeley – on behalf of a larger collaboration – report that an apparent supernova appeared at the site of the gamma-ray burst designated GRB 091127 (Troja et al. 2009, GCN 10191, posted at website URL http://gcn.gsfc.nasa.gov/gcn3/10191.gcn3), at position R.A. = 2h26m19s.9, Decl. = -18d57'09`` (equinox 2000.0), which is coincident with the optical afterglow (Immler et al. 2009, GCN 10193) of GRB 091127 at a redshift of z = 0.49 (Cucchiara et al. 2009, GCN 10202; Thoene et al. 2009, GCN 10233). Deep images obtained with the 8-m Gemini-South telescope on 2010 Mar. 10.1 UT show what is apparently a slightly elongated host galaxy, with the optical transient located 0’'.09 west and 0’'.26 south of the center of that galaxy, at a magnitude of I = 22.6. Optical imaging obtained using the Gemini Multi-Object Spectrograph on the same telescope and the ANDICAM instrument on the 1.3-m telescope at Cerro Tololo indicates that – while the optical afterglow of the GRB is initially dominant – the rising light from the presumed supernova becomes evident after about 10 days post-GRB, similar to SN 2003dh, associated with GRB 030329 (cf. IAUC 8114) becoming evident at about six days post-GRB (Bloom et al. 2004, Ap.J. 127, 252). The presumed supernova for GRB 091127 (here designated SN 2009nz) reached a peak brightness at about 22 +/- 3 days post-GRB, which is consistent with the rise-time expected for a GRB-supernova at redshift z = 0.49 [rise-time about 20*(1+z); e.g., Zeh et al. 2004, Ap.J. 609, 952]. In general, the rise and decay characteristics of SN 2009nz are similar to those of the prototypical GRB supernova, SN 1998bw. Gemini observations taken on 2009 Dec. 7.15 (about nine days post-burst) find the system to be at approximate I-magnitude 21.7, while extrapolations from the early-time afterglow decay suggest that an afterglow-only system at nine days post-burst would be around I-magnitude 22.1. From the first Gemini epoch to a second take on 2009 Dec. 16.03 (about 18 days post-burst), the system brightens by 0.06 +/- 0.02 magnitude. Without a supernova component, the afterglow should have dimmed during this time by nearly a magnitude. The first and second epoch are not simply detecting a host-galaxy component, however, because the decay of the supernova was then followed in a third epoch of Gemini imaging taken on 2010 Jan. 14.08 (about 47 days post-burst). From the second to the third epoch, the system dimmed by 0.58 +/- 0.02 magnitude. The observed peak magnitude of SN 2009nz (correcting for a small amount of Galactic reddening, A_V = 0.125, as well as the contribution from the host galaxy and optical afterglow) is I = 22.3. Assuming little or no host-galaxy reddening (which is consistent with the observations of the optical afterglow of the GRB), the absolute V magnitude of SN 2009nz is about -19.0, similar to SN 1998bw (cf. IAUC 6901), associated with GRB 980425 (Galama et al. 1998, Nature, 395, 670).