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, d_L=2.3 Gpc) in the Extended Groth Strip. It is located at a projected offset of ~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 ~ 0.13 keV and a 0.28-14.2 keV unabsorbed luminosity L_X ~ 4X10^{43} erg/s in the source rest frame. The source was still detected in three Chandra observations in 2002 August, with similarily ultrasoft but fainter spectra (kT_MCD ~ 0.17 keV, L_X ~ 0.5X10^{43} erg/s). 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 ~-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 ~10^5 Msun, assuming the peak X-ray luminosity at around the Eddington limit.
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