  We present improved values of the three components of the absolute space
velocity of the open cluster NGC6791. One HST ACS/WFC field with two-epoch
observations provides astrometric measurements of objects in a field containing
the cluster center. Identification of 60 background galaxies with sharp nuclei
allows us to determine an absolute reference point, and measure the absolute
proper motion of the cluster. We find (mu_alpha cos(delta), mu_delta)_J2000.0 =
(-0.57+/-0.13, -2.45+/-0.12)mas/yr, and adopt V_rad = -47.1+/-0.7km/s from the
average of the published values. Assuming a Galactic potential, we calculate
the Galactic orbit of the cluster for various assumed distances, and briefly
discuss the implications on the nature and the origin of this peculiar cluster.
