Multiwavelength flares from tidal disruption and accretion of stars can be used to find and study otherwise dormant massive black holes in galactic nuclei(1). Previous well-monitored candidate flares were short-lived, with most emission confined to within similar to 1 year(2-5). Here we report the discovery of a well-observed super-long (> 11 years) luminous X-ray flare from the nuclear region of a dwarf starburst galaxy. After an apparently fast rise within similar to 4 months a decade ago, the X-ray luminosity, though showing a weak trend of decay, has been persistently high at around the Eddington limit (when the radiation pressure balances the gravitational force). The X-ray spectra are soft - steeply declining towards higher energies and can be described with Comptonized emission from an optically thick low-temperature corona, a super-Eddington accretion signature often observed in accreting stellar-mass black holes(6). Dramatic spectral softening was also caught in one recent observation, implying either a temporary transition from the super-Eddington accretion state to the standard thermal state, or the presence of a transient highly blueshifted (similar to 0.36c) warm absorber. All these properties in concert suggest a tidal disruption event with an unusually long super-Eddington accretion phase that has never before been observed.