  We report the discovery of ASASSN-15lh (SN 2015L), which we interpret as the
most luminous supernova yet found. At redshift z = 0.2326, ASASSN-15lh reached
an absolute magnitude of M_{u,AB} = -23.5+/-0.1 and bolometric luminosity L_bol
= (2.2+/-0.2)x 10^45 ergs s^-1, which is more than twice as luminous as any
previously known supernova. It has several major features characteristic of the
hydrogen-poor super-luminous supernovae (SLSNe-I), whose energy sources and
progenitors are currently poorly understood. In contrast to most previously
known SLSNe-I that reside in star-forming dwarf galaxies, ASASSN-15lh appears
to be hosted by a luminous galaxy (M_K ~ -25.5) with little star formation. In
the 4 months since first detection, ASASSN-15lh radiated (1.1+/- 0.2)x10^52
ergs, challenging the magnetar model for its engine.
