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We report on a multi-wavelength study of the recently discovered X-ray transient XMMU J004243.6+412519 in M31, based on data collected with Swift and the 1.8-m Copernico Telescope at Cima Ekar in Asiago (Italy) between 2012 February and August. Undetected in all previous observations, in 2012 January XMMU J004243.6+412519 suddenly turned on, showing powerful X-ray emission with a luminosity of 1E+38 erg/s (assuming a distance of 780 kpc). In the following weeks, it reached a luminosity higher than 1E+39 erg/s, in the typical range of ultraluminous X-ray sources (ULXs). For at least 40 days the source luminosity remained fairly constant, then it faded below 1E+38 erg/s in the following 200 days. The source spectrum, which can be well described by multi-color disk blackbody model, progressively softened during the decay (the temperature changed from kT = 0.9 keV to 0.4 keV). No emission from XMMU J004243.6+412519 was detected down to 22 mag in the optical band and of 23-24 mag in the near ultraviolet. We compare the properties of XMMU J004243.6+412519 with those of other known ULXs and Galactic black hole transients, finding more similarities with the latter.