  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.
