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-Newton 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 } \approx 0.16 - 0.21 keV or a Wien spectrum Comptonized by an optically thick plasma with kT \approx 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 ) \times 10 ^ { 42 } erg s ^ { -1 } . Hard emission above \sim 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 \sim 10 ^ { 5 } M _ { \odot } ) .