Blood brother can refer to two or more people not related by birth who have sworn loyalty to each other. This is in modern times usually done in a ceremony, known as a blood oath, where each person makes a small cut, usually on a finger, hand, or the forearm, and then the two cuts are pressed together and bound, the symbolism being that each person's blood now flows in the other participant's veins.

The act carries a risk due to blood-borne diseases. The process usually provides a participant with a heightened symbolic sense of attachment with the other participant.

The Norsemen entering the pact of foster brotherhood (Icelandic: Fóstbræðralag) involved a rite in which they let their blood flow while they ducked underneath an arch formed by a strip of turf propped up by a spear or spears. An example is described in Gísla saga. In Fóstbræðra saga, the bond of Thorgeir Havarsson (Þorgeir Hávarsson) and Thormod Bersason (Þormóð Bersason) is sealed by such ritual as well, the ritual being called a leikr.

Örvar-Oddr's saga contains another notable account of blood brotherhood. Örvar-Oddr, after fighting the renowned Swedish warrior Hjalmar to a draw, entered a foster brotherhood with him by the turf-raising ritual. Afterwards, the strand of turf was put back during oaths and incantations.^{[citation needed]}

In the mythology of Northern Europe, Gunther and Högni became the blood brothers of Sigurd when he married their sister Gudrun. In Wagner's opera Götterdämmerung, the concluding part of his Ring Cycle, the same occurs between Gunther and Wagner's version of Sigurd, Siegfried, which is marked by the "Blood Brotherhood Leitmotiv". Additionally, it is briefly stated in Lokasenna that Odin and Loki are blood brothers.

Among the Scythians, the covenantors would allow their blood to drip into a cup; the blood was subsequently mixed with wine and drunk by both participants. Every man was limited to having three blood brotherhoods at any time lest his loyalties be distrusted. As a consequence, blood brotherhood was highly sought after and often preceded by a lengthy period of affiliation and friendship (Lucian, Toxaris). The 4th-century BC depictions of two Scythian warriors drinking from a single drinking horn (most notably in a gold appliqué from Kul-Oba) have been associated with the Scythian oath of blood brotherhood.

The Hungarian hajduks had a similar ceremony, but the wine was often replaced with milk so that the blood would be more visible.^{[citation needed]}

In Asian cultures, the act and the ceremony of becoming blood brothers is generally seen as a tribal relationship for bringing about alliance between tribes. It was practiced for that reason most notably by the Mongols, Turkic and early Chinese.^{[citation needed]}