An old boy network (also known as old boys' network, old boys' club) is an informal system in which wealthy men with similar social or educational backgrounds help each other in business or personal matters. The term originally referred to social and business connections among former pupils of male-only elite schools, though the term is now also used to refer to any closed system of relationships that restrict opportunities to within the group. The term originated from much of the British upper-class having attended certain fee-charging public schools as boys, thus former pupils are "old boys".

This can apply to the network between the graduates of a single school regardless of their gender. It is also known as an old boys' society and is similar to an alumni association. It can also mean a network of social and business connections among the alumni of various prestigious schools. In popular language, old boy network or old boys' society has come to be used for the preservation of social elites in general; such connections within the British Civil Service formed a primary theme in the BBC's satirical comedy series Yes Minister. The phrase "It's not what you know, it's who you know" is associated with this tradition.

In Australia, the term "Old Boy" is used to describe a male alumnus of some prestigious state and private schools. The term "Old Girl" is similarly used for a female alumna of such schools. Both "Old Girl" and "Old Boy" are sometimes used as a reference to someone's parents.

The term is also used in Canada, where the alumni of such schools as St. Andrew's College, Trinity College School, Crescent School, St George's School, Vancouver College, Stratford Hall, Bishop's College School, Hillfield Strathallan College, Collège Jean-de-Brébeuf, Lower Canada College, and Upper Canada College are known as Old Boys. Other influential private schools with powerful alumni networks may have become co-ed, such as Appleby College or University of Toronto Schools, but operate similarly in which large numbers of alumni all work for the same organization.

In Finland, the Finnish term hyvä veli -verkosto (literally dear brother network) is used to refer to the alleged informal network of men in high places whose members use their influence to pervert or circumvent official decision-making processes to the members' mutual benefit. As such, the term is pejorative.

The term derives from the salutation "Hyvä veli!", or "Dear brother!", traditionally used to open a letter to a not quite intimate friend. The implication is that since the elites of all fields are drawn from a fairly small pool of people who are mostly more or less acquainted with each other, they can and often do manage public and private affairs amongst themselves, off the record, and outside public scrutiny as they like. As the word "brother" implies, the network is usually presumed to consist of males, and thus the term is also sometimes used to refer to the marginalization of women and their exclusion from high positions in both the public and private spheres. There is an equivalent term, hyvä sisko ("dear sister"), used about informal networks of women in high positions.

President Urho Kekkonen was notable for directly communicating with senior officials (past his cabinets) through letters, which famously began with the salutation "Hyvä veli". These have been published in three volumes.

The term can also refer to the networks that are set up in the more elite secondary schools, such as Diocesan Boys' School, Queen's College, Ying Wa College, La Salle College and Saint Joseph's College.