In law a witness is someone who, either voluntarily or under compulsion, provides testimonial evidence, either oral or written, of what they know or claim to know.

A witness might be compelled to provide testimony in court, before a grand jury, before an administrative tribunal, before a deposition officer, or in a variety of other legal proceedings. A subpoena is a legal document that commands a person to appear at a proceeding. It is used to compel the testimony of a witness in a trial. Usually, it can be issued by a judge or by the lawyer representing the plaintiff or the defendant in a civil trial or by the prosecutor or the defense attorney in a criminal proceeding, or by a government agency. In many jurisdictions, it is compulsory to comply with the subpoena and either take an oath or solemnly affirm to testify truthfully under penalty of perjury.

Although informally a witness includes whoever perceived the event, in law, a witness is different from an informant. A confidential informant is someone who claimed to have witnessed an event or have hearsay information, but whose identity is being withheld from at least one party (typically the criminal defendant). The information from the confidential informant may have been used by a police officer or other official acting as a hearsay witness to obtain a search warrant.