  We present X-ray observations of the new transient magnetar Swift
J1834.9-0846, discovered with Swift BAT on 2011 August 7. The data were
obtained with Swift, RXTE, CXO, and XMM-Newton both before and after the
outburst. Timing analysis reveals singe peak pulsations with a period of 2.4823
s and an unusually high pulsed fraction, 85+/-10%. Using the RXTE and CXO data,
we estimated the period derivative, dot{P}=8\times 10^{-12} s/s, and confirmed
the high magnetic field of the source, B=1.4\times 10^{14} G. The decay of the
persistent X-ray flux, spanning 48 days, is consistent with a power law,
t^{-0.5}. In the CXO/ACIS image, we find that the highly absorbed point source
is surrounded by extended emission, which most likely is a dust scattering
halo. Swift J1834.9-0846 is located near the center of the radio supernova
remnant W41 and TeV source HESS J1834-087. An association with W41 would imply
a source distance of about 4 kpc; however, any relation to the HESS source
remains unclear, given the presence of several other candidate counterparts for
the latter source in the field. Our search for an IR counterpart of Swift
J1834.9-0846 revealed no source down to K_s=19.5 within the 0.6' CXO error
circle.
