We report the discovery of an off-nuclear ultrasoft hyper-luminous X-ray source candidate 3XMM J141711.1 + 522541 in the inactive S0 galaxy SDSS. J141711.07+522540.8 (z = 0.41827, dL = 2.3 Gpc) in the Extended Groth Strip. It is located at a projected offset of similar to 1.'' 0 (5.2 kpc) from the nucleus of the galaxy and was serendipitously detected in five XMM-Newton observations in 2000 July. Two observations have enough counts and can be fitted with a standard thermal disk with an apparent inner disk temperature kT(MCD) similar to 0.13 keV and a 0.28-14.2 keV unabsorbed luminosity LX similar to 4 x 10(43) erg s(-1) in the source rest frame. The source was still detected in three Chandra observations in 2002 August, with similarly ultrasoft but fainter spectra (kT(MCD) similar to 0.17 keV, LX similar to 0.5 x 10(43) erg s(-1)). It was not detected in later observations, including two by Chandra in 2005 October, one by XMM-Newton in 2014 January, and two by Chandra in 2014 September-October, implying a long-term flux variation factor of > 14. Therefore the source could be a transient with an outburst in 2000-2002. It has a faint optical counterpart candidate, with apparent magnitudes of m(F606W) = 26.3 AB mag and m(F814W) = 25.5 AB mag in 2004 December (implying an absolute V-band magnitude of similar to-15.9 AB mag). We discuss various explanations for the source and find that it is best explained as a massive black hole (BH) embedded in the nucleus of a possibly stripped satellite galaxy, with the X-ray outburst due to tidal disruption of a surrounding star by the BH. The BH mass is similar to 10(5)M circle dot, assuming the peak X-ray luminosity at around the Eddington limit.