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Given its peak luminosity and early-time spectra, ASASSN-15lh was classifiedas the most luminous supernova (SN) ever discovered (Dong et al. 2016). Here wereport a UV rebrightening of ASASSN-15lh observed with Swift during ourfollow-up campaign. The rebrightening began at $t \simeq$ 90 days (observerframe) after the primary peak and was followed by a $\sim 120$-day long plateauin the bolometric luminosity, before starting to fade again at $t\simeq 210$days. ASASSN-15lh rebrightened in the Swift UV bands by $\Delta m_{UVW2} \simeq-1.75$ mag, $\Delta m_{UVM2} \simeq -1.25$ mag, and $\Delta m_{UVW1} \simeq-0.8$ mag, but did not rebrighten in the optical bands. Throughout its initialdecline, subsequent rebrightening, and renewed decline, the spectra did notshow evidence of interactions between the ejecta and circumstellar medium (CSM)such as narrow emission lines. There are hints of weak H$\alpha$ emission atlate-times, but Margutti et al. (2016) have shown that it is narrow lineemission consistent with star formation in the host nucleus. By fitting ablackbody we find that during the rebrightening the effective photospherictemperature increased from $T_{BB} \simeq 11000$ K to $T_{BB} \simeq 18000$ K.Over the $\sim$ 550 days since its detection, ASASSN-15lh has radiated $\sim1.7 - 1.9 \times 10^{52}$ ergs. Although its physical nature remains uncertain,the evolution of ASASSN-15lh's photospheric radius, its radiated energy, andthe implied event rate, are all more similar to those of H-poor superluminoussupernovae (SLSNe-I) than to tidal disruption events (TDEs).

The Unexpected, Long-Lasting, UV Rebrightening of the Super-Luminous Supernova ASASSN-15lh