Res judicata is a Latin phrase meaning “a matter judged,” and in law it is the rule that once a court issues a final judgment on the merits of a dispute, the losing party cannot file a new lawsuit raising the same claim against the same opponent. The decision is permanent, and dissatisfaction with the result is not a basis for a second try.1Legal Information Institute. Res Judicata
The Three Elements That Trigger the Bar
A party invoking res judicata, almost always the defendant in the second lawsuit, has to establish three things. If any one of them is missing, the new case goes forward. If all three are present, the court dismisses it.
A Final Judgment on the Merits
The first case has to have ended with a decision that actually resolved the substance of the dispute. A jury verdict qualifies. So does summary judgment, because the court ruled on the legal and factual issues. A settlement that produces a dismissal “with prejudice” counts too, since both sides agreed the matter is closed for good.
Procedural dismissals do not count. Under Federal Rule of Civil Procedure 41(b), a dismissal for lack of jurisdiction, improper venue, or failure to join a required party is not treated as a judgment on the merits.2Legal Information Institute. Federal Rules of Civil Procedure Rule 41 – Dismissal of Actions The plaintiff can refile after fixing the problem. Voluntary dismissals are generally without prejudice the first time, but if a plaintiff voluntarily dismisses the same claim a second time, that second dismissal functions as a judgment on the merits and blocks any future suit.
Appeals complicate the picture. Federal courts have traditionally treated a trial judgment as final even while an appeal is pending, but some state courts hold the opposite view and say preclusion does not attach until appeals are exhausted. If a judgment is on appeal, which court system you are in determines whether it already has preclusive effect.
The Same Parties, or Parties in Privity
The second element requires that the people on both sides of the new lawsuit match the people from the first, either literally or through a close legal connection known as privity. In Taylor v. Sturgell, the Supreme Court identified the recognized categories: someone who agreed to be bound by the earlier result, someone with a pre-existing legal relationship to an original party (such as a buyer who stepped into a seller’s rights), someone whose interests were adequately represented in the first case (as with class members), someone who actually controlled the earlier litigation behind the scenes, someone now suing as the agent of a party who lost, and situations covered by a special statutory scheme like bankruptcy or probate.3Legal Information Institute. Taylor v. Sturgell
The Court in Taylor also rejected a looser idea some lower courts had used called “virtual representation,” which had sometimes bound non-parties simply because an earlier litigant had similar interests. That is no longer enough. A non-party is bound only through one of the recognized categories.
The Same Claim
The third element is the one that catches most people off guard. Courts generally apply the “transactional test” from the Restatement (Second) of Judgments, which treats a “claim” as covering every legal theory arising from the same transaction or connected series of events. The question is whether the facts are related in time, origin, and motivation, and whether they would logically be tried together.
You do not get to split one dispute into separate lawsuits based on different theories. If you hire a contractor, sue for breach of contract over shoddy work, and lose, you cannot turn around and sue the same contractor for fraud or negligence based on the same work. All of those theories grew out of the same transaction, and the time to raise them was in the first case. By not doing so, you forfeited them.1Legal Information Institute. Res Judicata This is what makes claim preclusion so unforgiving: it bars not just arguments you made but arguments you could have made.
Claim Preclusion and Issue Preclusion Are Not the Same Thing
When lawyers use “res judicata” loosely, they usually mean claim preclusion, which blocks an entire lawsuit. The broader doctrine also includes issue preclusion, historically called collateral estoppel, which is narrower and operates differently.
Issue preclusion prevents a specific factual or legal question from being relitigated, even in a lawsuit that is otherwise about something else. Four conditions apply: the earlier judgment must be valid and final, the exact same issue must arise again, that issue must have been actually litigated and decided the first time, and the court’s resolution of it must have been essential to the judgment.4Legal Information Institute. Issue Preclusion
A practical example: a court decides in a contract case that a particular signature on a document was forged. If those same parties later end up in a different lawsuit where the same signature’s authenticity matters, the forgery finding is locked in. The rest of the new case proceeds, but that question is settled.5Legal Information Institute. Collateral Estoppel
Issue preclusion can be used defensively, by a defendant stopping a plaintiff from relitigating something the plaintiff already lost,6Legal Information Institute. Defensive Collateral Estoppel or offensively, by a new plaintiff holding a defendant to something that defendant lost against someone else. The Supreme Court permitted the offensive use in Parklane Hosiery Co. v. Shore but gave trial judges discretion to refuse it when fairness is a concern, such as when the plaintiff could have joined the first case and simply waited to benefit, or when the defendant had little incentive to fight hard the first time.7Justia. Parklane Hosiery Co., Inc. v. Shore, 439 U.S. 322 (1979)
How the Defense Gets Raised
Res judicata does not kick in on its own. A court will not scan its own docket, notice a duplicate case, and dismiss it. The defendant has to raise the doctrine as an affirmative defense. Federal Rule of Civil Procedure 8(c)(1) requires that it be specifically asserted in the answer to the complaint.8Legal Information Institute. Federal Rules of Civil Procedure Rule 8 – General Rules of Pleading Skip that step and the defense can be waived, even when all three elements are plainly satisfied.
In practice, the defendant then files a motion to dismiss or a motion for summary judgment, attaching records from the earlier case to show the judgment was final, the parties match, and the claim grew out of the same transaction. The burden sits entirely on the defendant. If the evidence of the prior judgment’s scope and finality falls short, the new case moves forward.
Judgments From Other Courts Still Count
A judgment from one court system generally has to be respected by every other. Under 28 U.S.C. § 1738, properly authenticated judicial proceedings from any state court are entitled to the same preclusive effect in every other court in the country that they would get in the state where they were entered.9Office of the Law Revision Counsel. 28 U.S. Code 1738 – State and Territorial Statutes and Judicial Proceedings; Full Faith and Credit
One consequence trips up litigants: when a federal court is deciding whether a prior state judgment bars a new federal case, it applies the preclusion law of the state where that judgment was entered, not a federal standard. If the state defines “same claim” more narrowly, that narrower rule controls. State courts likewise have to give federal judgments their full preclusive effect. Moving a case into a different court system is not a way out.
When a Judgment Can Still Be Reopened
Finality matters, but it is not absolute. A short list of recognized exceptions allows a party to attack or sidestep a prior judgment when enforcing it would be genuinely unjust.
Void Judgments
A judgment issued by a court that lacked jurisdiction is void and has no preclusive force. Federal Rule of Civil Procedure 60(b)(4) lets a party move to set aside a void judgment with no time limit.10Office of the Law Revision Counsel. Federal Rules of Civil Procedure Rule 60 – Relief from Judgment or Order This usually comes up when the first court had no personal jurisdiction over the defendant or no subject matter jurisdiction over the kind of case.
Fraud or Misconduct
If a judgment was procured by fraud, misrepresentation, or serious misconduct by the opposing party, Rule 60(b)(3) allows the court to set it aside, so long as the motion is filed within one year.10Office of the Law Revision Counsel. Federal Rules of Civil Procedure Rule 60 – Relief from Judgment or Order Fraud on the court itself, such as bribing a judge, is treated more seriously and can be challenged through an independent action without a fixed time limit.
Other Grounds
Rule 60(b) also permits relief for newly discovered evidence that reasonable diligence could not have surfaced in time, for judgments that have been reversed or satisfied, and for situations where continued enforcement would be inequitable. The catch-all in Rule 60(b)(6), covering “any other reason justifying relief,” is treated as a safety valve for extraordinary circumstances outside the other categories.
It Is Not the Same as Double Jeopardy
Res judicata and double jeopardy both prevent a repeat of the same fight, but they operate in different areas of law. Res judicata is a civil doctrine that stops private parties from relitigating civil claims. Double jeopardy is a Fifth Amendment criminal protection that stops the government from prosecuting a person twice for the same offense after an acquittal or conviction.
The two can overlap at the edges. A criminal case and a civil case over the same underlying event are treated as different claims, so a criminal acquittal does not generally bar a later civil suit. But if a particular factual issue was actually decided in the criminal case, issue preclusion may lock that fact into the civil case. When criminal and civil proceedings run in parallel, the line between an entire claim and a single issue is where the practical consequences fall.