ATL12 contains along-track sea surface height (SSH) of the global open ocean relative to the WGS84 ellipsoid. Height averages, distributions, and uncertainties are provided over ocean segments 400 to 7,000 m in length. Heights relative to the EGM2008 geoid and variables such as photon return rate are provided in 10 m along-track bins within each ocean segment. Additionally, 10 m bins corresponding to ATL07 bright leads are identified, and heights within the bins are averaged over ocean segments to yield accurate SSHs where ice concentrations are greater than 15%. The data were acquired by the Advanced Topographic Laser Altimeter System (ATLAS) instrument on board the ICESat-2 observatory.