We present photometry and spectroscopy of PS1-14bj, a hydrogen-poor superluminous supernova (SLSN) at redshift z = 0.5215 discovered in the last months of the Pan-STARRS1 Medium Deep Survey. PS1-14bj stands out because of its extremely slow evolution, with an observed rise of greater than or similar to 125 rest-frame days, and exponential decline out to similar to 250 days past peak at a measured rate of 0.01 mag day(-1), consistent with fully trapped Co-56 decay. This is the longest rise time measured in an SLSN to date, and the first SLSN to show a rise time consistent with pair-instability supernova (PISN) models. Compared to other slowly evolving SLSNe, it is spectroscopically similar to the prototype SN 2007bi at maximum light, although lower in luminosity (L-peak similar or equal to 4.6 x 10(43) erg s(-1) ) and with a flatter peak than previous events. PS1-14bj shows a number of peculiar properties, including a near-constant color temperature for > 200 days past peak, and strong emission lines from [O III] lambda 5007 and [O III] lambda 4363 with a velocity width of similar to 3400 km s(-1) in its late-time spectra. These both suggest there is a sustained source of heating over very long timescales, and are incompatible with a simple Ni-56-powered/PISN interpretation. A modified magnetar model including emission leakage at late times can reproduce the light curve, in which case the blue continuum and [O III] features are interpreted as material heated and ionized by the inner pulsar wind nebula becoming visible at late times. Alternatively, the late-time heating could be due to interaction with a shell of H-poor circumstellar material.