Vexatious litigation is legal action which is brought solely to harass or subdue an adversary. It may take the form of a primary frivolous lawsuit or may be the repetitive, burdensome, and unwarranted filing of meritless motions in a matter which is otherwise a meritorious cause of action. Filing vexatious litigation is considered an abuse of the judicial process and may result in sanctions against the offender.

A single action, even a frivolous one, is usually not enough to raise a litigant to the level of being declared vexatious. Rather, a pattern of frivolous legal actions is typically required to rise to the level of vexatious. Repeated and severe instances by a single lawyer or firm can result in eventual disbarment.

Some jurisdictions have a list of vexatious litigants: people who have repeatedly abused the legal system. Because lawyers could be disbarred for participating in this abuse of the legal process, vexatious litigants are often unable to retain legal counsel, and such litigants, therefore, represent themselves in court. Those on the vexatious litigant list are usually either forbidden from any further legal action or are required to obtain prior permission from a senior judge before taking any legal action. The process by which a person is added to the list varies among jurisdictions. In liberal democratic jurisdictions, declaring someone a vexatious litigant is considered to be a serious measure and rarely occurs, as judges and officials are reluctant to curtail a person's access to the courts.

These legal actions occur in some countries of the former British Empire, where the common law system still remains: Australia, Canada, Ireland, New Zealand, UK, and US, which are specified below. The civil (codified/continental) law does not have such a prohibition.