Subject-matter jurisdiction (also called jurisdiction ratione materiae) is the authority of a court to hear cases of a particular type of cases relating to a specific subject matter. For instance, bankruptcy court only has the authority to hear bankruptcy cases.

Subject-matter jurisdiction must be distinguished from personal jurisdiction, which is the power of a court to render a judgment against a particular defendant, and territorial jurisdiction, which is the power of the court to render a judgment concerning events that have occurred within a well-defined territory. Unlike personal or territorial jurisdiction, lack of subject-matter jurisdiction cannot be waived. A judgment from a court that did not have subject-matter jurisdiction is forever a nullity. To decide a case, a court must have a combination of subject (subjectam) and either personal (personam) or territorial (locum) jurisdiction.

Subject-matter jurisdiction, personal or territorial jurisdiction, and adequate notice are the three most fundamental constitutional requirements for a valid judgment.

In United States law, federal question jurisdiction is a type of subject-matter jurisdiction that gives United States federal courts the power to hear civil cases where the plaintiff alleges a violation of the United States Constitution, federal law, or a treaty to which the United States is a party. The federal question jurisdiction statute is codified at 28 U.S.C. § 1331.

The district courts shall have original jurisdiction of all civil actions arising under the Constitution, laws, or treaties of the United States.

— 28 U.S.C. § 1331.

In the law of the United States, diversity jurisdiction is a form of subject-matter jurisdiction that gives U.S. federal courts the power to hear lawsuits that do not involve a federal question. For a U.S. federal court to have diversity jurisdiction over a lawsuit, two conditions must be met. First, there must be "diversity of citizenship" between the parties, meaning the plaintiffs must be citizens of different U.S. states than the defendants. Second, the lawsuit's "amount in controversy" must be more than $75,000. If a lawsuit does not meet these two conditions, U.S. federal courts will normally lack the power to hear it unless it involves a federal question, and the lawsuit would need to be heard in state court instead.

The United States Constitution, in Article 3, Section 2, grants Congress the power to permit federal courts to hear diversity cases through legislation authorizing such jurisdiction. The provision was included because the Framers of the Constitution were concerned that when a case is filed in one state, and it involves parties from that state and another state, the state court might be biased toward the party from that state. Congress first exercised that power and granted federal trial circuit courts diversity jurisdiction in the Judiciary Act of 1789. Diversity jurisdiction is currently codified at 28 U.S.C. § 1332.

In 1969, the American Law Institute explained in a 587-page analysis of the subject that diversity is the "most controversial" type of federal jurisdiction, because it "lays bare fundamental issues regarding the nature and operation of our federal union."