In ancient Rome, contubernium was a quasi-marital relationship between two slaves or between a slave (servus) and a free person who was usually a former slave or the child of a former slave. A slave involved in such a relationship was called contubernalis, the basic and general meaning of which was "companion".

Under Roman law, the slave was treated as property (res) and lacked the legal personhood to enter into legitimate forms of marriage. Although not codified as marriage (conubium) in Roman law, contubernium had legal implications that were addressed by Roman jurists in case law and was intended to be a lasting, ideally permanent union modelled on marital affection (affectio maritalis) that was approved and recognized by the slave's owner. Inscriptions indicate that contubernium with the intentions it expressed was primarily a concern for "upwardly mobile" slaves and former slaves who served the imperial house (familia Caesaris) and bureaucracy or who belonged to houses of senatorial or equestrian rank. In contubernium when only one partner remained enslaved, most often the free person was a freedwoman (liberta).

Contubernium institutionalized the slave's honorable intention to form an enduring heterosexual union with economic, emotional, and parental benefits—an acknowledgment of the inherent contradictions between the status of the slave as property and "tool" (instrumentum) and the individual's evident humanity and desire to participate in what Romans regarded as normal family life.

The word contubernalis seems sometimes to have been used as an enduring term of endearment even after a formerly enslaved couple achieved a legal status that allowed them to marry formally.

Men who commemorate the death of a female partner set up epitaphs for a contubernali bene merenti ("to a well deserving companion"), carissimae ("dearest"), piissimae or pientissimae ("most devoted"), optimae ("best"), incomparabili ("incomparable"), rarissimae ("most rare"), sanctissimae ("most sacred"), and dulcissimae ("sweetest"). Women also use many of these descriptors for the male partner they lost, and the same adjectives are used in epitaphs for spouses who had legal marriages.

Roman law regarded the slave as property; slaves lacked legal personhood and therefore could not enter into contracts on their own behalf, including legally sanctioned forms of marriage (conubium). Some slaves, however, were permitted or encouraged to form family units—permanent heterosexual unions within which "natural children" (liberi naturales) might be reared as vernae. Roman jurists themselves when discussing case law sometimes refer to unions involving slave contubernales with terms for "husband" and "wife". In inscriptions, most contubernales whose status can be identified are slaves, though liberti and libertae (freedpersons) are also frequent. Although a slave lacked legal standing to contract a marriage recognized by Roman law, slaves who came from outside Italy would have their own marriage rites and customs, and there was no prohibition against slaves considering themselves married by custom in this way.

On a country estate (villa), a male slave who had proven his reliability might have license to cohabit with a fellow female slave (conserva). The agricultural writer Columella says that contubernium is particularly desirable for the vilicus, the bailiff or overseer of a farm, who was often but not always a slave or former slave. Any children born from these unions would increase the master's wealth. Vernae, slaves who had been born and reared within the household of their enslaved mother, were themselves more likely to be allowed to cohabit as a couple and to rear their own children. Inscriptions reinforce the impression that contubernium was a privilege extended to slaves whose work at higher-skill jobs was valued.

It was possible for contubernium to exist between two slaves with different owners, but the clearest evidence occurs within a household. During most of the Republican and Imperial eras, free Roman women had the right as citizens to own property and retained their property rights within marriage. The mistress of the house would have her own staff, and many of the household's female slaves are likely to have been hers. Documented examples of couples in which each is affiliated with a different gens (extended family or clan) suggest that the two slaves might still belong to the same household or estate, one in the possession of the male head of household as dominus and the other of his wife as domina.