Fraternization (from Latin frater meaning "brother") is the act of establishing intimate relations between people or groups. It is generally used to refer to establishing relations that are considered ethically or morally wrong or inappropriate.

In many institutional contexts (such as militaries, diplomatic corps, parliaments, prisons, law enforcement or police, schools, sports teams, gangs and corporations) fraternization transgresses legal, moral, or professional norms forbidding certain categories of social contact across socially or legally defined classes. The term often tends to connote impropriety, unprofessionalism or a lack of ethics.

For example, "fraternization with the enemy" refers to associations with members of enemy groups and suggests a serious conflict of strong, deep, and close romantic interest and attraction, if not the possibility of treason, "fraternization with civilians" typically suggests transgression of norms forbidding non-civilians and civilians from forming close nonprofessional relationships (such as romantically or platonically); and "fraternization of officers with enlisted personnel" or "seniors with their juniors" (the usual meaning in a military context) describes associations that are implied to be irregular, unprofessional, improper, or imprudent in ways that negatively affect the members and goals of the organization.

Many institutions worldwide implement policies forbidding forms of fraternization for many specific reasons. Fraternization may be forbidden to maintain image and morale, to protect and ensure fair and uniform treatment of subordinates, to maintain organizational integrity and the ability to achieve operational goals, and to prevent unauthorized transfers of information. Relations and activities forbidden under anti-fraternization policies may be romantic and sexual liaisons, gambling and ongoing business relationships, insubordination, or excessive familiarity and disrespect of rank.

Views on fraternization are mixed and may depend on the relations and classes under discussion. Organizations may relax, change, or reinforce restrictions to reflect changes in the prevailing organizational view or doctrine regarding fraternization. Within an in-group fraternization may lead to friendship and self actualization.