Res judicata is the rule that once a court has issued a final judgment on the merits of a dispute, the losing party cannot sue again over the same claim. The doctrine, also called claim preclusion, protects defendants from being hauled back into court over a matter already decided and keeps courts from spending resources on disputes that have already had their day. It reaches further than many people expect: it blocks not only the exact lawsuit that was filed before, but also any legal theory arising from the same underlying events that could have been raised and was not.
The Three Requirements
A court will apply res judicata only when three conditions are present together.
First, there must be a final judgment on the merits from the earlier case. That means the court actually decided the dispute based on the facts and the law, not that it dismissed the case for a procedural problem. Jury verdicts qualify. So do directed verdicts, where a judge concludes the evidence is so one-sided that no reasonable jury could rule differently. Summary judgments also count, since the court has determined there is no genuine factual dispute and one side wins as a matter of law.1Legal Information Institute. Federal Rules of Civil Procedure Rule 56 – Summary Judgment A judgment becomes final when the trial court has finished its work and nothing is left but enforcement. Most jurisdictions treat the trial court’s order as final for preclusion purposes even while an appeal is pending, which stops a losing party from filing a brand-new case elsewhere as a workaround.
Second, the second lawsuit must involve the same claim as the first. This is where most fights over res judicata happen, and it has its own section below.
Third, the lawsuits must involve the same parties, or people legally close enough to count through a doctrine called privity.
Underlying all three requirements is a basic prerequisite: the first court had to have proper jurisdiction. If the original court lacked personal jurisdiction over the parties or subject matter jurisdiction over the dispute, its judgment is void and blocks nothing. A small claims court with a dollar cap, for instance, cannot produce a binding judgment on a contract claim far exceeding that limit.
What Counts as the Same Claim
Modern courts almost universally use the transactional test drawn from the Restatement (Second) of Judgments. Under this approach, a claim includes all rights to remedies arising out of the same transaction or connected series of transactions. Courts decide the boundaries of a transaction pragmatically, looking at whether the facts are related in time, space, and origin, whether they would form a convenient trial unit, and whether treating them as a unit matches the parties’ expectations.
Applied to a familiar example: if a car accident causes both physical injuries and property damage, both arise from the same transaction. You cannot sue for the property damage first, collect, and then file a separate lawsuit for your injuries. Both claims come from the same event, and the doctrine demands that you bring everything at once. Any legal theory you could have raised but held back is permanently extinguished when the first case ends. Plaintiffs who save a theory for strategic reasons learn this the hard way.
A small number of jurisdictions still use the older “same evidence” test, which asks whether identical proof would support both claims. That standard is narrower, but the clear trend has moved toward the transactional approach, and federal courts apply it almost universally.
Not every follow-on lawsuit between the same parties triggers the doctrine. If a landlord and tenant litigate over unpaid rent for one lease period, that judgment does not block a later suit over unpaid rent for a different period. The parties and legal theory are the same, but the operative facts are different.
Compulsory Counterclaims
The same logic cuts in the other direction for defendants. Under Federal Rule of Civil Procedure 13(a), if you are sued and you have a claim against the plaintiff arising from the same transaction, you must raise it as a counterclaim in that lawsuit.2Legal Information Institute. Federal Rules of Civil Procedure Rule 13 – Counterclaim and Crossclaim Skip it, and you lose the right to bring that claim later. This is one of the most commonly overlooked traps in civil litigation. Defendants focused entirely on fighting off the plaintiff’s claims sometimes forget they had an offensive claim of their own that needed to go in the same case.
Who Gets Bound: Parties and Privity
Res judicata binds only the parties who appeared in the original lawsuit and those in privity with them. A complete stranger to the first case, who never had a chance to present their side, cannot be blocked by its outcome. Binding nonparties without their participation would run into due process protections.
Privity exists when a nonparty’s legal interests are closely enough linked to an original party’s that treating them as the same person is fair. The most common scenario involves property transfers. If a court rules on a boundary dispute and the property later changes hands, the new owner is bound by that ruling because the legal right attaches to the land, not the person who happened to own it during the lawsuit. Legal representatives create privity too: a trustee acting for beneficiaries or an executor managing an estate can bind the people they represent. The same logic applies when a company finances and directs an employee’s defense; the company cannot later disavow the result by claiming it was not a named party.
The Six Categories From Taylor v. Sturgell
The Supreme Court drew firm boundaries around nonparty preclusion in Taylor v. Sturgell (2008), rejecting the loose theory of “virtual representation” that some lower courts had used to bind nonparties whenever someone with similar interests had already litigated. The Court held that nonparties can be bound only under six recognized categories:3Legal Information Institute. Taylor v. Sturgell
- Agreement to be bound. A nonparty who consents in advance to be bound by another lawsuit’s outcome is held to that agreement.
- A pre-existing substantive legal relationship, such as assignor and assignee, that carries preclusive consequences built into the relationship itself.
- Adequate representation in the earlier suit by someone with the same interests, in limited circumstances.
- Control over the earlier litigation. A nonparty who assumed control over how the case was litigated is bound by its result.
- Proxy litigation. A party already bound by a judgment cannot dodge preclusion by sending an agent or representative to relitigate.
- A special statutory scheme, such as bankruptcy, that expressly forecloses successive litigation by nonparties.
Outside these categories, courts cannot bind someone simply because a person with similar interests already litigated and lost.
How a Defendant Raises Res Judicata
Res judicata does not kick in automatically. The Federal Rules of Civil Procedure classify it as an affirmative defense under Rule 8(c), which means the defendant must raise it in the answer to the complaint.4Legal Information Institute. Federal Rules of Civil Procedure Rule 8 – General Rules of Pleading Fail to assert it, and you risk waiving the defense entirely. Sitting on the defense and springing it months later tends to get the motion denied.
In practice, a defendant who believes a prior judgment bars the plaintiff’s claims will plead the defense in the answer and then file a motion to dismiss or a motion for summary judgment supported by evidence of the earlier case. That evidence usually includes the prior judgment, the docket records, and the pleadings from the first action. The court then reviews whether all requirements are met and whether the current claims fall within the scope of what was already decided.
Res Judicata vs. Collateral Estoppel
Res judicata is frequently confused with collateral estoppel, also called issue preclusion. The two doctrines have different triggers and different reach.
Claim preclusion blocks an entire claim. Once a final judgment resolves a dispute, you cannot bring the same cause of action again. Issue preclusion is narrower: it prevents relitigation of a specific factual or legal question that was already decided, even when the second lawsuit involves a completely different claim. If a court determines in a breach-of-contract case that a particular signature is a forgery, a later fraud case between the same parties cannot relitigate whether that signature was genuine. The issue was already resolved.
The key word separating the two doctrines is “actually.” Res judicata bars claims that were raised or could have been raised. Collateral estoppel bars only issues that were actually litigated and actually decided. If an issue could have come up in the first case but nobody raised it, collateral estoppel does not apply, though res judicata might still bar the whole claim that would have contained it.
The two doctrines also differ on who can invoke them. Res judicata generally requires the same parties on both sides. Some courts, by contrast, allow issue preclusion to be invoked by someone who was not a party to the first case. A new plaintiff can use “offensive” nonmutual issue preclusion to stop a defendant from relitigating a fact the defendant already lost on. That tool has limits: the Supreme Court has held it does not apply against the federal government, and courts will reject it when earlier judgments on the same issue have been inconsistent.
When Res Judicata Does Not Apply
Even when the parties and claims look identical, several situations can keep a prior judgment from blocking a new lawsuit.
The most fundamental exception applies when the first court lacked subject matter jurisdiction. A judgment from a court without authority over the type of dispute is a legal nullity. This commonly comes up when a state court rules on a matter reserved exclusively for federal courts, with patent infringement and bankruptcy as classic examples.
Certain dismissals also do not trigger preclusion because the court never evaluated the actual dispute. Under the federal rules, dismissals for lack of jurisdiction, improper venue, failure to join a required party, voluntary dismissals, and dismissals expressly labeled “without prejudice” are not adjudications on the merits.5Legal Information Institute. Federal Rules of Civil Procedure Rule 19 – Required Joinder of Parties The plaintiff in these situations can fix the procedural defect and refile.
Due process failures defeat preclusion too. If a party never received proper notice of the original lawsuit, the resulting judgment cannot bind them regardless of what it decided. A judgment entered against someone who had no real opportunity to participate is constitutionally infirm.
Relief From Judgment Under Rule 60(b)
Federal Rule of Civil Procedure 60(b) provides a mechanism for reopening a final judgment after entry, which effectively removes the judgment’s preclusive effect if the motion succeeds. The rule lists several grounds:6U.S. District Court for the Northern District of Illinois. Federal Rule of Civil Procedure 60 – Relief From Judgment or Order
- Mistake, inadvertence, surprise, or excusable neglect by the court, counsel, or a party.
- Newly discovered evidence that could not have been found in time despite reasonable diligence.
- Fraud or misconduct by the opposing party, including concealment of evidence.
- A void judgment entered without jurisdiction or in violation of due process.
- Changed circumstances, such as the judgment being satisfied, a prior judgment it relied on being reversed, or continued enforcement becoming inequitable.
- A catch-all for any other extraordinary reason justifying relief.
Timing matters. For the first three grounds, mistake, new evidence, and fraud, the motion must be filed within one year of the judgment. For the remaining grounds, courts require only that the motion be filed within a reasonable time, a deliberately vague standard that depends on the circumstances. Filing a Rule 60(b) motion does not by itself suspend the judgment or pause enforcement; the judgment remains in effect unless and until the court grants relief.
Judgments From Other Jurisdictions
A judgment does not lose its preclusive force at the state line. Under 28 U.S.C. ยง 1738, federal courts and courts in other states must give a final judgment the same preclusive effect it would receive in the state that issued it.7Office of the Law Revision Counsel. 28 U.S. Code 1738 – State and Territorial Statutes and Judicial Proceedings; Full Faith and Credit If the first state’s preclusion rules would bar relitigation, courts in the second state must honor that result, even if they would apply a different standard to their own judgments. A losing party cannot cross a border to escape the reach of claim preclusion.