Frivolous litigation is the use of legal processes with apparent disregard for the merit of one's own arguments. It includes presenting an argument with reason to know that it would certainly fail, or acting without a basic level of diligence in researching the relevant law and facts. That a claim was lost does not imply the claim in itself was frivolous.

Frivolous litigation may be based on absurd legal theories, may involve a superabundance or repetition of motions or additional suits, may be uncivil or harassing to the court, or may claim extreme remedies. A claim or defense may be frivolous because it had no underlying justification in fact, or because it was not presented with an argument for a reasonable extension or reinterpretation of the law. A claim may be deemed frivolous because existing laws unequivocally prohibit such a claim, such as a so-called Good Samaritan law.

In the United States, Rule 11 of the Federal Rules of Civil Procedure and similar state rules require that an attorney perform a due diligence investigation concerning the factual basis for any claim or defense. Jurisdictions differ on whether a claim or defense can be frivolous if the attorney acted in good faith. Because such a defense or claim wastes the court's and the other parties' time, resources and legal fees, sanctions may be imposed by a court upon the party or the lawyer who presents the frivolous defense or claim. The law firm may also be sanctioned, or even held in contempt.