  We report the discovery of a 200mHz quasi-periodic oscillation (QPO) in the
X-ray emission from a bright ultra-luminous X-ray source (ULX) Holmberg IX X-1
using a long XMM-Newton observation. The QPO has a centroid at
202.5_{-3.8}^{+4.9}mHz, a coherence Q ~9.3 and an amplitude (rms) of 6% in the
0.2-10keV band. This is only the second detection of a QPO from an ULX, after
M82 X-1, and provides strong evidence against beaming. The power spectrum is
well fitted by a power law with an index of ~0.7. The total integrated power
(rms) is ~9.4% in the 0.001-1Hz range. The X-ray spectrum shows clear evidence
for a soft X-ray excess component that is well described by a multicolor disk
blackbody (kT_in ~ 0.3keV) and a high energy curvature that can be modeled
either by a cut-off power law (Gamma ~ 1; E_cutoff ~9keV) or as a strongly
Comptonized continuum in an optically thick (tau ~7.3) and cool (kT_e ~3keV)
plasma. Both the presence of the QPO and the shape of the X-ray spectrum
strongly suggest that the ULX is not in the high/soft or thermally dominated
state. A truncated disk and inner optically thick corona may explain the
observed X-ray spectrum and the presence of the QPO.
