  We report the discovery with the EPIC CCD cameras onboard XMM-Newton of a 54
mHz quasiperiodic oscillation (QPO) in the > 2 keV X-ray flux from an
ultraluminous X-ray source (ULX) in the starburst galaxy M82. This is the first
detection of a QPO in the X-ray flux from an extra-Galactic ULX, and confirms
that the source is a compact object. Based on the QPO strength and previous
Chandra observations it appears likely that the QPO is associated with the most
luminous object in the central region of M82, CXOM82 J095550.2+694047, however,
XMM imaging alone is not sufficient to unambiguously confirm this. The other
plausible candidate is CXOM82 J095551.1+694045, however, the QPO luminosity is
comparable to the peak luminosity of this object in Chandra observations, which
argues against it being the source of the QPO. The QPO had a centroid frequency
of 54.3 +- 0.9 mHz, a coherence of 5, and an amplitude (rms) in the 2 - 10 keV
band of 8.5%. Below 0.2 Hz the power spectrum can be described by a power-law
with index ~1, and amplitude (rms) of 13.5%. The X-ray spectrum requires a
curving continuum, with a disk-blackbody (diskbb) at T = 3.1 keV providing an
acceptable fit. A broad Fe line centered at 6.55 keV is required in all fits,
but the equivalent width (EW) is sensitive to the continuum model. There is no
evidence of a reflection component. The implied bolometric luminosity is 4 - 5
x 10^{40} ergs/sec. Archival Rossi X-ray Timing Explorer (RXTE) pointings at
M82 also show evidence for QPOs in the 50 - 100 mHz frequency range. We discuss
the implications of our findings for models of ULXs.
