Semtek v. Lockheed Martin, 531 U.S. 497 (2001), is a United States Supreme Court case in which the Court held that the claim preclusive effect of a federal judgment on a claim over which subject matter jurisdiction is based solely on diversity is determined by the common law of the state in which the federal district court rendering the decision is located.

Petitioners Semtek International Incorporated filed a complaint against Lockheed Martin in California state court, alleging a breach of contract.

The trial judge then dismissed the complaint, writing that California's 2-year statute of limitations made the claim "barred".

The court here also dismissed the complaint, but on the grounds that "the res judicata effect" precludes this separate claim in a different state.

Scalia wrote that there was no final "judgment on the merits" in the California case and thus the Maryland trial was not precluded.