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We introduce the  GALEX  Arecibo SDSS Survey (GASS), an on‐going large programme that is gathering high quality H  i ‐line spectra using the Arecibo radio telescope for an unbiased sample of ∼1000 galaxies with stellar masses greater than  10 10  M ⊙   and redshifts  0.025 &lt;  z  &lt; 0.05 , selected from the Sloan Digital Sky Survey (SDSS) spectroscopic and  Galaxy Evolution Explorer  ( GALEX ) imaging surveys. The galaxies are observed until detected or until a low gas mass fraction limit (1.5–5 per cent) is reached. This paper presents the first Data Release, consisting of ∼20 per cent of the final GASS sample. We use this data set to explore the main scaling relations of the H  i  gas fraction with galaxy structure and  NUV− r   colour. A large fraction (∼60 per cent) of the galaxies in our sample are detected in H  i . Even at stellar masses above  10 11  M ⊙  , the detected fraction does not fall below ∼40 per cent. We find that the atomic gas fraction   M  H  i  / M  ★   decreases strongly with stellar mass, stellar surface mass density and  NUV− r   colour, but is only weakly correlated with the galaxy bulge‐to‐disc ratio (as measured by the concentration index of the  r ‐band light). We also find that the fraction of galaxies with significant (more than a few per cent) H  i  decreases sharply above a characteristic stellar surface mass density of  10 8.5  M ⊙  kpc −2  . The fraction of gas‐rich galaxies decreases much more smoothly with stellar mass. One of the key goals of GASS is to identify and quantify the incidence of galaxies that are  transitioning  between the blue, star‐forming cloud and the red sequence of passively evolving galaxies. Likely transition candidates can be identified as outliers from the mean scaling relations between   M  H  i  / M  ★   and other galaxy properties. We have fitted a plane to the two‐dimensional relation between the H  i  mass fraction, stellar surface mass density and  NUV− r   colour. Interesting outliers from this plane include gas‐rich red sequence galaxies that may be in the process of regrowing their discs, as well as blue, but gas‐poor spirals.

The GALEX Arecibo SDSS Survey – I. Gas fraction scaling relations of massive galaxies and first data release