Res judicata or res iudicata, also known as claim preclusion, is the Latin term for judged matter, and refers to either of two concepts in common law civil procedure: a case in which there has been a final judgment and that is no longer subject to appeal; and the legal doctrine meant to bar (or preclude) relitigation of a claim between the same parties.

In the case of res judicata, the matter cannot be raised again, either in the same court or in a different court. A court will use res judicata to deny reconsideration of a matter. The doctrine of res judicata is a method of preventing injustice to the parties of a completed case, and avoiding waste of judicial resources. Res judicata does not merely prevent future judgments from contradicting earlier ones, but also prevents litigants from multiplying judgments.

It is similar to the concept of double jeopardy and non bis in idem in criminal law, but the protection in criminal prosecutions only bars an identical prosecution for the same offense. However, a different offense may be charged on identical evidence at a second trial; whereas, res judicata precludes any causes of action or claims that may arise from the previously litigated subject matter.

In common law jurisdictions, the principle of res judicata may be asserted either by a judge or a defendant.

Once a final judgment has been handed down in a lawsuit, subsequent judges who are confronted with a suit that is identical to or substantially the same as the earlier one will apply the res judicata doctrine to preserve the effect of the first judgment.

A defendant in a lawsuit may use res judicata as defense. The general rule is that a plaintiff who prosecuted an action against a defendant and obtained a valid final judgment is not able to initiate another action against the same defendant where:

For example, once a bankruptcy plan is confirmed in a court action, the plan is binding on all parties involved. Any question regarding the plan which could have been raised but was not may be barred by res judicata.

The Seventh Amendment to the United States Constitution provides that no fact having been tried by a jury shall be otherwise re-examinable in any court of the United States or of any state than according to the rules of law.