A legal fiction is a construct used in law where, in order to facilitate a specific outcome, a position is taken to be true, even if such a position is not literally true. Legal fictions can be employed by the courts or found in legislation.

Legal fictions are different from legal presumptions which assume a certain state of facts until the opposite is proved, such as the presumption of legitimacy.

The term legal fiction is sometimes used in a pejorative way. Jeremy Bentham was a famous historical critic of legal fictions. Proponents of legal fictions, particularly of their use historically, identify legal fictions as "scaffolding around a building under construction".

Child adoption is a legal fiction in that the adoptive parents become the legal parents, notwithstanding the lack of a biological relationship. Once an order or judgment of adoption is entered, the (presumptive) biological parents become legal strangers to the child, legally no longer related nor with any rights related to the child. Conversely, the adoptive parents are legally considered to be parents of the adopted child. A new birth certificate reflecting this is issued, which is a legal fiction.^{[citation needed]}

If two or more people die within a period of time or in a manner that renders it impossible to tell the order in which they died, the older of the two is considered to have died first. This is in order to safeguard the operation of certain general legal rules, e.g. in inheritance law, where the younger person will inherit the older, hence being able to pass on. If a parent dies alongside a child, who has a child of their own, the rule of the elder predeceasing the child will allow the grandchild (typically) to inherit both, the parent directly and grandparent indirectly, with the parent instantaneously inheriting and then bequeathing.

The doctrine of survival, although still existing in England, has been abolished in many U.S. states by the Uniform Simultaneous Death Act.

The common law had a procedure whereby title to land could be put in direct issue, called the "writ of right". The defendant could insist on trial by "wager of battle", that is trial by combat, a judicially sanctioned duel. To avoid the plaintiff staking life and limb, a tale was told in the pleadings about how one John Doe leased land from the plaintiff but was ousted by Richard Roe, who claimed a contrary lease from the defendant. Such events would lead to the "mixed action in ejectment", a procedure to determine title via trial by jury. This is the origin of the names John Doe and Richard Roe for anonymous parties. The fiction of Doe, Roe, and the leases was not challenged by the parties unless they wished to stake their life on a trial by combat. Wager of battle fell into disuse by the end of the thirteenth century though it was not abolished in England until 1819.

In cases where the court must determine whether a standard has been reached, such as whether a defendant has been negligent, the court frequently uses the legal fiction of the "reasonable person". This is known as the "objective test", and is far more common than the "subjective test" where the court seeks the viewpoint of the parties (or "subjects"). Sometimes, the court may apply a "mixed test", as in the House of Lords' decision in DPP v Camplin 1978.