We present the first observations of a Type I superluminous supernova (SLSN) at≳ 1000 days after maximum light. We observed SN 2015bn using the Hubble Space Telescope (HST) Advanced Camera for Surveys in the F475W, F625W and F775W filters at 721 days and 1068 days. SN 2015bn is clearly detected and resolved from its compact host, allowing reliable photometry. A galaxy template constructed from these data further enables us to isolate the SLSN flux in deep ground-based imaging. We measure a light curve decline rate at> 700 days of 0.19±0.03 mag (100 d)− 1, much shallower than the earlier evolution, and slower than previous SLSNe (at any phase) or the decay rate of 56 Co. Neither additional radioactive isotopes nor a light echo can consistently account for the slow decline. A spectrum at 1083 days shows the same [O i] λ6300 and [Ca ii] λ7300 lines as seen at∼ 300–400 days, with no new features to indicate strong circumstellar interaction. Radio limits with the Very Large Array rule out an extended wind for mass-loss rates