An amicus curiae (lit. 'friend of the court'; PL amici curiae) is an individual or organization who is not a party to a legal case, but who is permitted to assist a court by offering information, expertise, or insight that has a bearing on the issues in the case. The decision on whether to consider an amicus brief lies within the discretion of the court. 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.