Perjury 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. In some jurisdictions, contrary to popular misconception, 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. For example, it is not perjury 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 the United States, for example, 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 as well as most Australian states. In Queensland, under Section 124 of the Queensland Criminal Code Act 1899, perjury is punishable by up to life in prison if it is committed to procure an innocent person for a crime that is punishable by life in prison. However, prosecutions for perjury are rare. In some countries such as France and Italy, suspects cannot be heard under oath or affirmation and so cannot commit perjury, regardless of what they say during their trial.

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. Federal tax law provides criminal penalties of up to three years in prison for violation of the tax return perjury statute.

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. 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.

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.