Vitamin B₁₂, also called cobalamin, is a water-soluble vitamin that has a key role in the normal functioning of the nervous system via the synthesis of myelin (myelinogenesis), and in the maturation of developing red blood cells in the bone marrow. It is involved in the metabolism of every cell of the human body: it is a cofactor in DNA synthesis, fatty acid metabolism, and amino acid metabolism.

No fungi, plants, or animals (including humans) are capable of synthesizing vitamin B₁₂: only bacteria and archaea have the enzymes needed for its synthesis. Other sources are foods with the vitamin artificially added ("fortified"), intramuscular injection, and vitamin B₁₂ tablets ("supplements") taken either orally or sublingually.

Vitamin B₁₂ is one of eight B vitamins; it is the largest and most structurally complicated vitamin. It consists of a class of chemically related compounds (vitamers), all of which show pharmacological activity. It contains the biochemically rare element cobalt (chemical symbol Co) positioned in the center of a planar tetra-pyrrole ring called a corrin ring. The vitamer is produced by bacteria as hydroxocobalamin, but conversion among different forms of the vitamin occurs in the body after consumption. It is produced industrially via bacterial fermentation and also via Vitamin B₁₂ total synthesis.

Vitamin B₁₂ was discovered as a result of its relationship to the disease pernicious anemia, an autoimmune disease in which parietal cells of the lining of the stomach are destroyed: parietal cells are responsible for secreting both intrinsic factor and hydrochloric acid into the stomach. Because intrinsic factor is crucial for the normal absorption of B₁₂, its lack in the presence of pernicious anemia causes a vitamin B₁₂ deficiency. Many other subtler kinds of vitamin B₁₂ deficiency and their biochemical effects have since been made clear. Due to impairment of vitamin B₁₂ absorption during aging, people over age 60 are at risk of deficiency.