Deuterium (or hydrogen-2, symbol ²
H
or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one neutron, whereas the far more common protium has no neutrons in the nucleus. Deuterium has a natural abundance in Earth's oceans of about one atom of deuterium among every 6,420 atoms of hydrogen (see heavy water). Thus deuterium accounts for approximately 0.0156% by number (0.0312% by mass) of all the naturally occurring hydrogen in the oceans, while protium accounts for more than 99.98%. The abundance of deuterium changes slightly from one kind of natural water to another (see Vienna Standard Mean Ocean Water). (Tritium is yet another hydrogen isotope with symbol ³
H
or T. It has two neutrons, and is radioactive and far more rare than deuterium.)

The name deuterium is derived from the Greek deuteros, meaning "second", to denote the two particles composing the nucleus. Deuterium was discovered by American chemist Harold Urey in 1931. Urey and others produced samples of heavy water in which the deuterium content had been highly concentrated. The discovery of deuterium won Urey a Nobel Prize in 1934.

Deuterium is destroyed in the interiors of stars faster than it is produced. Other natural processes are thought to produce only an insignificant amount of deuterium. Nearly all deuterium found in nature was produced in the Big Bang 13.8 billion years ago, as the basic or primordial ratio of hydrogen-1 to deuterium (about 26 atoms of deuterium per million hydrogen atoms) has its origin from that time. This is the ratio found in the gas giant planets, such as Jupiter. The analysis of deuterium–protium ratios in comets found results very similar to the mean ratio in Earth's oceans (156 atoms of deuterium per million hydrogen atoms). This reinforces theories that much of Earth's ocean water is of cometary origin. The deuterium–protium ratio of the comet 67P/Churyumov–Gerasimenko, as measured by the Rosetta space probe, is about three times that of Earth water. This figure is the highest yet measured in a comet.

Deuterium–protium ratios thus continue to be an active topic of research in both astronomy and climatology.