The pater familias, also written as paterfamilias (plural patres familias), was the head of a Roman family. The pater familias was the oldest living male in a household, and could legally exercise autocratic authority over his extended family. The term is Latin for "father of the family" or the "owner of the family estate". The form is archaic in Latin, preserving the old genitive ending in -ās (see Latin declension), whereas in classical Latin the normal first declension genitive singular ending was -ae. The pater familias always had to be a Roman citizen.

Roman law and tradition (mos majorum) established the power of the pater familias within the community of his own extended familia. In Roman family law, the term "Patria potestas" (Latin: “power of a father”) refers to this concept. He held legal privilege over the property of the familia, and varying levels of authority over his dependents: these included his wife and children, certain other relatives through blood or adoption, clients, freedmen and slaves. The same mos majorum moderated his authority and determined his responsibilities to his own familia and to the broader community. He had a duty to father and raise healthy children as future citizens of Rome, to maintain the moral propriety and well-being of his household, to honour his clan and ancestral gods and to dutifully participate—and if possible, serve—in Rome's political, religious and social life. In effect, the pater familias was expected to be a good citizen. In theory at least, he held powers of life and death over every member of his extended familia through ancient right. In practice, the extreme form of this right was seldom exercised. It was eventually limited by law.

In the Roman tradition, the term has appeared mostly in legal texts, and to a lesser extent, in literary texts. In both types of discourses, the term has been most commonly used to refer to the “estate owner,” a title considered conceptually separate from his familial relations.