We describe the ROSAT International X-ray/Optical Survey (RIXOS), a medium-sensitivity survey and optical identification of X-ray sources discovered in ROSAT high Galactic latitude fields (|b|>28 deg) and observed with the Position Sensitive Proportional Counter (PSPC) detector. The survey made use of the central 17arcmin of each ROSAT field. A flux limit of 3x10-14ergcm-2s-1 (0.5-2keV) was adopted for the survey, and a minimum exposure time of 8000s was required for qualifying ROSAT observations. X-ray sources in the survey are therefore substantially above the detection threshold of each field used, and many contain enough counts to allow the X-ray spectral slope to be estimated. Spectroscopic observations of potential counterparts were obtained of all sources down to the survey limit in 64 fields, totalling a sky area of 15.77deg2. Positive optical identifications are made for 94per cent of the 296 sources thus examined. A further 18 fields (4.44deg2), containing 105 sources above the 3x10-14ergcm-2s-1 survey limit, are completely optically identified to a higher flux of 8x10-14ergcm-2s-1 (0.5-2keV). Optical spectroscopic data are supplemented by deep CCD imaging of many sources to reveal the morphology of the optical counterparts, and objects too faint to register on Sky Survey plates. The faintest optical counterparts have R~22. This paper describes the survey method, and presents a catalogue of the RIXOS sources and their optical identifications. Finding charts based on Sky Survey data are given for each source, supplemented by CCD imaging where necessary.