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 $5sigma$ $AB$ depths in an aperture of $2arcsec$ diameter of $sim 25$ in $Y$ and $sim 24$ in $JHK_{rm s}$ bands and all have sub-arcsecond seeing. To this $5sigma$ 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 $1sigma$ astrometric RMS in both directions for stars selected with $17.0<K_{rm s}rm {(AB)} <19.5$ is $sim 0.08arcsec$ 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 ESOs phase 3 archiving and distribution system and from the UltraVISTA web site.
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