A sham marriage or fake marriage is a marriage of convenience entered into without intending to create a real marital relationship. This is usually for the purpose of gaining an advantage from the marriage.

Definitions of sham marriage vary by jurisdiction, but are often related to poverty. The essential point in the varying definitions is whether the couple intend to live in a real marital relationship, to establish a life together. A typical definition by the UK Home Office in 2015:

A sham marriage or civil partnership is one where the relationship is not genuine but one party hopes to gain an immigration advantage from it. There is no subsisting relationship, dependency, or intent to live as husband and wife or civil partners.

While referred to as a "sham" or "fake" because of its motivation, the union itself is legally valid if it conforms to the formal legal requirements for marriage in the jurisdiction. Arranging or entering into such a marriage to deceive public officials is in itself a violation of the law of some countries, for example the US.

After a period, couples often divorce if there is no purpose in remaining married. The reverse situation, in which a couple gets a divorce while continuing to live together, is called paper divorce.

Sham marriages are sometimes considered distinct from a marriage fraud, which is a type of romance scam, in which one spouse is unwittingly taken advantage of by the foreign spouse who feigns romantic interest, typically in order to obtain a residence permit or for money.

Common reasons for sham marriages are to gain immigration, residency, work, or citizenship rights for one of the spouses.

There have been cases of people entering into a sham marriage to avoid suspicion of homosexuality, bisexuality, etc. For example, Hollywood studios had allegedly requested homosexual/homoromantic actors, such as Rock Hudson, to conceal their homosexuality in a so-called lavender marriage.