Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the International Atomic Energy Agency (based in Vienna) in 1968, and since 1993 continues to be evaluated and studied by the IAEA along with the European Institute for Reference Materials and Measurements and the American National Institute of Standards and Technology. The standard includes both the established values of stable isotopes found in waters and calibration materials provided for standardization and interlaboratory comparisons of instruments used to measure these values in experimental materials.

The designation ocean water refers to water molecules collected directly from the ocean, rather than from other points in the water cycle (e.g., rain, snow, river or lake water). Water from different points in the water cycle contains molecules with differing ratios of isotopes due to the slight differences the isotopes cause in the rates of evaporation and condensation. VSMOW is not a standard for seawater, which contains other molecules in addition to water molecules. Fresh distilled VSMOW is used to make high accuracy measurements of water's physical properties and to produce laboratory standards.

Before VSMOW was defined, average ocean water and melted snow were used as references. These conventions were refined in the 1960s by the standardized definition of Standard Mean Ocean Water (SMOW). The U.S. National Bureau of Standards (now the National Institute of Standards and Technology, NIST) created physical water standards for global use. However, the physical integrity of the U.S. standards came into question. The use of the SMOW standard was discontinued.

VSMOW is a recalibration of the original SMOW definition and was created in 1967 by Harmon Craig and other researchers from Scripps Institution of Oceanography at the University of California, San Diego who mixed distilled ocean waters collected from different spots around the globe. VSMOW remains one of the major isotopic water benchmarks in use today, used for both hydrogen and oxygen isotopes.