Oxytocin (Oxt) ( /ˌɒksɨˈtoʊsɪn/) is a mammalian hormone that acts primarily as a neuromodulator in the brain.

Oxytocin is best known for its roles in sexual reproduction, in particular during and after childbirth. It is released in large amounts after distension of the cervix and uterus during labor, facilitating birth, and after stimulation of the nipples, facilitating breastfeeding.

Recent studies have begun to investigate oxytocin's role in various behaviors, including orgasm, social recognition, pair bonding, anxiety, and maternal behaviors. For this reason, it is sometimes referred to as the "love hormone." The inability to secrete oxytocin and feel empathy is linked to sociopathy, psychopathy, narcissism^{[citation needed]} and general manipulativeness.

The word oxytocin was derived from the Greek ὼκυτοκίνη, ōkytokínē, meaning “quick birth”, after its uterine-contracting properties were discovered by British pharmacologist Sir Henry Hallett Dale in 1906. The milk ejection property of oxytocin was described by Ott and Scott in 1910 and by Schafer and Mackenzie in 1911. The nine amino acid sequence of oxytocin was elucidated by Vincent du Vigneaud et al. and by Tuppy in 1953. and synthesized biochemically soon after by du Vigneaud et al. in 1953. Oxytocin was the very first polypeptide hormone to be sequenced and synthesized. Du Vigneaud states in his publication of 1954 "This synthesis thus constitutes the first synthesis of a polypeptide hormone".