An amicus curiae (lit. 'friend of the court'; pl. amici curiae) is an individual or organization that is not a party to a legal case, but that is permitted to assist a court by offering information, expertise, or insight that has a bearing on the issues in the case. Whether an amicus brief will be considered is typically under the court's discretion. The phrase is legal Latin and the origin of the term has been dated to 1605–1615. The scope of amici curiae is generally found in the cases where broad public interests are involved and concerns regarding civil rights are in question.

In American law, an amicus curiae typically refers to what in some other jurisdictions is known as an intervenor: a person or organization who requests to provide legal submissions so as to offer a relevant alternative or additional perspective regarding the matters in dispute. In the American courts, the amicus may be referred to as an amicus brief. In other jurisdictions, such as Canada, an amicus curiae is a lawyer who is asked by the court to provide legal submissions regarding issues that would otherwise not be aired properly, often because one or both of the parties is not represented by counsel.

In international courts, legal submissions by intervenors are called amicus curiae observations.

Direct or indirect connections between the amicus curiae figure and the Roman juridical experience are still debated. Some scholars simply explain the Latin expression with the fact that the cultural elites' (including the jurists') language of the Anglo-Saxon world was Latin, so that many Latin legal terms first spread through English law, and then also in the Law of the United States. The Italian academic Giovanni Criscuoli, while admitting the theoretical possibility of eventually comparing it with the Roman figure of the "consiliarius", concludes that: "it is a figure of exclusive Anglo-Saxon blood".

Starting in the 9th century, it was incorporated into English law, and it was later extended to most common law systems. Later, it was introduced in international law, in particular concerning human rights. From there, it was integrated in some civil law systems (it has been, as at 2013, integrated into Argentina's law system and Honduras's 2010 civil procedures code). Today, it is used by the European Court of Human Rights, the Inter-American Commission on Human Rights, the Inter-American Court of Human Rights, the Court of Justice of the European Union and the Special Tribunal for Lebanon.

The role of an amicus was described by Lord Justice of Appeal Cyril Salmon in Allen v Sir Alfred McAlpine & Sons Ltd [1968] 2 QB 229 at p. 266 F-G:

I had always understood that the role of an amicus curiae was to help the court by expounding the law impartially, or if one of the parties were unrepresented, by advancing the legal arguments on his behalf.

The situation most often noted in the press is when an advocacy group files a brief in a case before an appellate court in which it is not a litigant. Appellate cases are normally limited to the factual record and arguments coming from the lower court case under appeal; attorneys focus on the facts and arguments most favorable to their clients. Where a case may have broader implications, amicus curiae briefs are a way to articulate those concerns, so that the possibly broad legal or public policy implications of the court's anticipated decisions will not depend solely on the positions and arguments advanced by the parties directly involved in the case.