Supplemental jurisdiction refers to the authority exercised by the United States federal courts to hear additional claims which are substantially related to the original claim in a suit; even though the court lacks subject-matter jurisdiction to hear the additional claims independently. It provides a U.S. federal court the jurisdiction over claims that do not have independent federal subject matter jurisdiction, but are closely related to a claim that does have federal jurisdiction.
Supplemental Jurisdiction is a common-law device that allows a court to resolve all claims between opposing parties in one forum. 28 U.S.C. § 1367 provides that a federal court hearing a federal claim can also hear substantially related state law claims, thereby encouraging efficiency by only having one trial at the federal level rather than one trial in federal court and another in state court. However, if the case is brought as a diversity action, generally, no supplemental jurisdiction is entertained if such claims would destroy complete diversity. Supplementary jurisdiction is discretionary and a federal court can choose whether or not to exercise it in a given case.
For example, if a tenant sued a landlord in federal court alleging he was discriminated against in violation of the Fair Housing Act; that lawsuit would come under the “federal question” jurisdiction. In addition, if the tenant also complained that the landlord damaged his car by negligently breaking a window, this claim would not normally come under the jurisdiction of a federal court. However, a federal court could opt to exercise its “supplemental jurisdiction” and hear this claim along with the Fair Housing Act claim.