Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of surface varies according to the measurement method used, but it is between 1 millimetre (0.04 in) and 20 metres (70 ft) below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a short distance of the shore. Localized areas of heavy snow can form in bands downwind of warm water bodies within an otherwise cold air mass. Warm sea surface temperatures are known to be a cause of tropical cyclogenesis over the Earth's oceans. Tropical cyclones can also cause a cool wake, due to turbulent mixing of the upper 30 metres (100 ft) of the ocean. SST changes diurnally, like the air above it, but to a lesser degree. There is less SST variation on breezy days than on calm days. In addition, ocean currents such as the Atlantic Multidecadal Oscillation (AMO), can affect SST's on multi-decadal time scales, a major impact results from the global thermohaline circulation, which affects average SST significantly throughout most of the world's oceans.

Coastal SSTs can cause offshore winds to generate upwelling, which can significantly cool or warm nearby landmasses, but shallower waters over a continental shelf are often warmer. Onshore winds can cause a considerable warm-up even in areas where upwelling is fairly constant, such as the northwest coast of South America. Its values are important within numerical weather prediction as the SST influences the atmosphere above, such as in the formation of sea breezes and sea fog. It is also used to calibrate measurements from weather satellites.

It is very likely that global mean sea surface temperature increased by 0.88°C between 1850-1900 and 2011-2020 due to global warming, with most of that warming (0.60°C) occurring between 1980 and 2020.^: 1228 Land surface temperatures have increased faster than ocean temperatures as the ocean absorbs about 92% of excess heat generated by climate change.