The interaction of a supernova with a circumstellar medium (CSM) can dramatically increase the emitted luminosity by converting kinetic energy to thermal energy. In superluminous supernovae (SLSNe) of Type IIn -- named for narrow hydrogen lines in their spectra -- the integrated emission can reach $sim 10^{51}$ erg, attainable by thermalising most of the kinetic energy of a conventional SN. A few transients in the centres of active galaxies have shown similar spectra and even larger energies, but are difficult to distinguish from accretion onto the supermassive black hole. Here we present a new event, SN2016aps, offset from the centre of a low-mass galaxy, that radiated $gtrsim 5 times 10^{51}$ erg, necessitating a hyper-energetic supernova explosion. We find a total (SN ejecta $+$ CSM) mass likely exceeding 50-100 M$_odot$, with energy $gtrsim 10^{52}$ erg, consistent with some models of pair-instability supernovae (PISNe) or pulsational PISNe -- theoretically-predicted thermonuclear explosions from helium cores $>50$ M$_odot$. Independent of the explosion mechanism, this event demonstrates the existence of extremely energetic stellar explosions, detectable at very high redshifts, and provides insight into dense CSM formation in the most massive stars.
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