Fratricide (from Latin fratricidium; from frater 'brother' and -cīdium 'killing' – the assimilated root of caedere 'to kill, cut down') is the act of killing one's own brother.

It can either be done directly or via the use of either a hired or an indoctrinated intermediary (an assassin). The victim need not be the perpetrator's biological brother. In a military context, fratricide refers to a service member killing a comrade.

The term is often used metaphorically to refer to civil wars.

The Abrahamic religions recognize the biblical account of Cain and Abel as the first fratricidal murder to be committed. Esau swore to kill Jacob after Jacob stole his blessings, and later, Jacob's sons planned to kill Joseph, but instead sold him. Judge Gideon's son, Avimelech, killed his seventy brothers leaving the youngest, Jotham.

In the mythology of ancient Rome, the city is founded as the result of a fratricide, with the twins Romulus and Remus quarreling over who has the favour of the gods and over each other's plans to build Rome, with Romulus becoming Rome's first king and namesake after killing his brother.

In Greek mythology, Cassiphone, the daughter of Odysseus and Circe, killed her half-brother and husband Telemachus in revenge after he killed her mother Circe following a quarrell between the two.

In ancient Egyptian mythology, the god Osiris is murdered by his brother Set who usurps the throne.

In the Hindu epic Mahābhārata, Karna was killed by Arjuna who was unaware that Karna was his eldest brother.