  We report the discovery of a candidate active galactic nucleus (AGN), 2XMM
J123103.2+110648 at z = 0.13, with an X-ray spectrum represented purely by soft
thermal emission reminiscent of Galactic black hole (BH) binaries in the
disk-dominated state. This object was found in the second XMM serendipitous
source catalogue as a highly variable X-ray source. In three separate
observations, its X-ray spectrum can be represented either by a multicolor disk
blackbody model with an inner temperature of kT_in~0.16-0.21 keV or a Wien
spectrum Comptonized by an optically thick plasma with kT~0.14-0.18 keV. The
soft X-ray luminosity in the 0.5--2 keV band is estimated to be (1.6-3.8)x10^42
erg/s. Hard emission above ~2 keV is not detected. The ratio of the soft to
hard emission is the strongest among AGNs observed thus far. Spectra selected
in high/low flux time intervals are examined in order to study spectral
variability. In the second observation with the highest signal-to-noise ratio,
the low energy (below 0.7 keV) spectral regime flattens when the flux is high,
while the shape of the high energy part (1-1.7 keV) remains unchanged. This
behavior is qualitatively consistent with being caused by strong
Comptonization. Both the strong soft excess and spectral change consistent with
Comptonization in the X-ray spectrum imply that the Eddington ratio is large,
which requires a small BH mass (smaller than ~10^5M_solar.

High Energy Astrophysical Phenomena