Hydrogen-poor superluminous supernovae (SLSN-I) are a class of rare and energetic explosions that have been discovered in untargeted transient surveys in the past decade^,. The progenitor stars and the physical mechanism behind their large radiated energies (about 10⁵¹ erg or 10⁴⁴ J) are both debated, with one class of models primarily requiring a large rotational energy^, and the other requiring very massive progenitors that either convert kinetic energy into radiation through interaction with circumstellar material (CSM)^{, , –} or engender an explosion caused by pair-instability (loss of photon pressure due to particle–antiparticle production)^,. Observing the structure of the CSM around SLSN-I offers a powerful test of some scenarios, although direct observations are scarce^,. Here, we present a series of spectroscopic observations of the SLSN-I iPTF16eh, which reveal both absorption and time- and frequency-variable emission in the Mg ii resonance doublet. We show that these observations are naturally explained as a resonance scattering light echo from a circumstellar shell. Modelling the evolution of the emission, we infer a shell radius of 0.1 pc and velocity of 3,300 km s⁻¹, implying that the shell was ejected three decades before the supernova explosion. These properties match theoretical predictions of shell ejections occurring because of pulsational pair-instability and imply that the progenitor had a helium core mass of about 50–55 M_⊙, corresponding to an initial mass of about 115 M_⊙.