  We report the discovery of a light echo (LE) from the Type Ia supernova (SN)
2006X in the nearby galaxy M100. The presence of the LE is supported by
analysis of both the Advanced Camera for Surveys (ACS) images taken with the
{\it Hubble Space Telescope (HST)} at $\sim$300 d after maximum brightness and
the Keck optical spectrum obtained at a similar phase. In the image procedure,
both the radial-profile analysis and the point-spread-function (PSF)
subtraction method resolve significant excess emission at 2--5 ACS pixels
($\sim0.05''-0.13''$) from the center. In particular, the PSF-subtracted ACS
images distinctly appear to have an extended, ring-like echo. Due to
limitations of the image resolution, we cannot confirm any structure or flux
within 2 ACS pixels from the SN. The late-time spectrum of SN 2006X can be
reasonably fit with two components: a nebular spectrum of a normal SN Ia and a
synthetic LE spectrum. Both image and spectral analysis show a rather blue
color for the emission of the LE, suggestive of a small average grain size for
the scattering dust. Using the Cepheid distance to M100 of 15.2 Mpc, we find
that the dust illuminated by the resolved LE is $\sim$27--170 pc from the SN.
The echo inferred from the nebular spectrum appears to be more luminous than
that resolved in the images (at the $\sim2\sigma$ level), perhaps suggesting
the presence of an inner echo at $<$2 ACS pixels ($\sim0.05''$). It is not
clear, however, whether this possible local echo was produced by a distinct
dust component (i.e., the local circumstellar dust) or by a continuous, larger
distribution of dust as with the outer component. Nevertheless, our detection
of a significant echo in SN 2006X confirms that this supernova was produced in
a dusty environment having unusual dust properties.
