We report on the discovery of a periodic modulation in the bright supersoft X-ray source XMMU J004252.5+411540 detected in the 2000-2004 XMM-Newton observations of M31 .
The source exhibits X-ray pulsations with a period P \sim 217.7 s and a quasi-sinusoidal pulse shape and pulsed fraction \sim 7-11 % .
We did not detect statistically significant changes in the pulsation period on the time scale of 4 years .
The X-ray spectra of XMMU J004252.5+411540 are extremely soft and can be approximated with an absorbed blackbody of temperature 62-77 eV and a weak power law tail of photon index \Gamma \sim 1.7-3.1 in the 0.2-3.0 keV energy band .
The X-ray properties of the source and the absence of an optical/UV counterpart brighter than 19 mag suggest that it belongs to M31 .
The estimated bolometric luminosity of the source varies between \sim 2 \times 10 ^ { 38 } and \sim 8 \times 10 ^ { 38 } ergs s ^ { -1 } at 760 kpc , depending on the choice of spectral model .
The X-ray pulsations and supersoft spectrum of XMMU J004252.5+411540 imply that it is almost certainly an accreting white dwarf , steadily burning hydrogen-rich material on its surface .
We interpret X-ray pulsations as a signature of the strong magnetic field of the rotating white dwarf .
Assuming that the X-ray source is powered by disk accretion , we estimate its surface field strength to be in the range 4 \times 10 ^ { 5 } G < B _ { 0 } < 8 \times 10 ^ { 6 } G. XMMU J004252.5+411540 is the second supersoft X-ray source in M31 showing coherent pulsations , after the transient supersoft source XMMU J004319.4+411758 with 865.5 s pulsation period .