Perjury (also known as foreswearing) is the intentional act of swearing a false oath or falsifying an affirmation to tell the truth, whether spoken or in writing, concerning matters material to an official proceeding.

Like most other crimes in the common law system, to be convicted of perjury one must have had the intention (mens rea) to commit the act and to have actually committed the act (actus reus). Further, statements that are facts cannot be considered perjury, even if they might arguably constitute an omission, and it is not perjury to lie about matters that are immaterial to the legal proceeding. Statements that entail an interpretation of fact are not perjury because people often draw inaccurate conclusions unwittingly or make honest mistakes without the intent to deceive. Individuals may have honest but mistaken beliefs about certain facts or their recollection may be inaccurate, or may have a different perception of what is the accurate way to state the truth. In some jurisdictions, no crime has occurred when a false statement is (intentionally or unintentionally) made while under oath or subject to penalty. Instead, criminal culpability attaches only at the instant the declarant falsely asserts the truth of statements (made or to be made) that are material to the outcome of the proceeding. It is not perjury, for example, to lie about one's age except if age is a fact material to influencing the legal result, such as eligibility for old age retirement benefits or whether a person was of an age to have legal capacity.

Perjury is considered a serious offense, as it can be used to usurp the power of the courts, resulting in miscarriages of justice. In Canada, those who commit perjury are guilty of an indictable offence and liable to imprisonment for a term not exceeding fourteen years. Perjury is a statutory offence in England and Wales. A person convicted of perjury is liable to imprisonment for a term not exceeding seven years, or to a fine, or to both. In the United States, the general perjury statute under federal law classifies perjury as a felony and provides for a prison sentence of up to five years. The California Penal Code allows for perjury to be a capital offense in cases causing wrongful execution. Perjury which caused the wrongful execution of another or in the pursuit of causing the wrongful execution of another is respectively construed as murder or attempted murder, and is normally itself punishable by execution in countries that retain the death penalty. Perjury is considered a felony in most U.S. states. However, prosecutions for perjury are rare.

The rules for perjury also apply when a person has made a statement under penalty of perjury even if the person has not been sworn or affirmed as a witness before an appropriate official. An example is the US income tax return, which, by law, must be signed as true and correct under penalty of perjury (see 26 U.S.C. § 6065). Federal tax law provides criminal penalties of up to three years in prison for violation of the tax return perjury statute. See: 26 U.S.C. § 7206(1)

In the United States, Kenya, Scotland and several other English-speaking Commonwealth nations, subornation of perjury, which is attempting to induce another person to commit perjury, is itself a crime.