Supplemental jurisdiction allows the federal court to hear additional claims that it would not independently have subject matter jurisdiction (either diversity jurisdiction or federal question jurisdiction) over. These claims must be closely related to at least one of the existing claims that the federal court does have an original subject matter jurisdiction over. If the additional claims and the base claim arise out of a “common nucleus of operative fact,” then the court may (but does not have to) exercise supplemental jurisdiction to hear the other claims as well.
The specific rules are later codified in 28 USC §1367. Generally, the base claim and the supplemental claim need to be “so related…that they form part of the same case or controversy.” Moreover, if the base claim is one of diversity jurisdiction, the court shall not exercise supplemental jurisdiction over claims:
- By an original plaintiff against persons joining under Rule 14, 19, 20, or 24 of the Federal Rules of Civil Procedure (i.e. through impleader, mandatory or permissive joinder, or intervention), or
- By persons proposed to be joined as plaintiffs under Rule 19 or 24 (i.e. through mandatory joinder or intervention).
Finally, the court may decline to exercise supplemental jurisdiction if:
- The claim raises a novel or complex issue of state law,
- The claim substantially predominates over the claim or claims over which the district court has original jurisdiction,
- The district court has dismissed all claims over which it has original jurisdiction, or
- In exceptional circumstances, there are other compelling reasons for declining jurisdiction.
[Last updated in June of 2024 by the Wex Definitions Team]