  In this paper we describe the first data release of the UltraVISTA
near-infrared imaging survey of the COSMOS field. We summarise the key goals
and design of the survey and provide a detailed description of our data
reduction techniques . We provide stacked, sky-subtracted images in $YJHK_{\rm
s}$ and narrow-band filters constructed from data collected during the first
year of UltraVISTA observations. Our stacked images reach $5\sigma$ $AB$ depths
in an aperture of $2\arcsec$ diameter of $\sim 25$ in $Y$ and $\sim 24$ in
$JHK_{\rm s}$ bands and all have sub-arcsecond seeing. To this $5\sigma$ limit,
our $K_{\rm s}$ catalogue contains 216,268 sources. We carry out a series of
quality assessment tests on our images and catalogues, comparing our stacks
with existing catalogues. The $1\sigma$ astrometric RMS in both directions for
stars selected with $17.0<K_{\rm s}\rm {(AB)} <19.5$ is $\sim 0.08\arcsec$ in
comparison to the publicly-available COSMOS ACS catalogues. Our images are
resampled to the same pixel scale and tangent point as the publicly available
COSMOS data and so may be easily used to generate multi-colour catalogues using
this data. All images and catalogues presented in this paper are publicly
available through ESO's "phase 3" archiving and distribution system and from
the UltraVISTA web site.
