Res judicata: when a prior judgment kills the new lawsuit

A plaintiff who lost a case the first time around does not always go quietly. Sometimes they refile under a new theory. Sometimes they sue a different but related defendant. Sometimes they wait years and try again with a different lawyer. For defendants, res judicata is the tool that ends those second acts before they get started. Done right, the motion takes a single certified copy of a prior judgment and turns it into a dismissal with prejudice.

This guide walks through the elements of claim preclusion as the federal courts apply them, the major Supreme Court decisions that define its scope, and the briefing structure that turns a complicated doctrine into a clean motion. The key insight is that res judicata is not just about identical lawsuits. It is about the entire transaction or series of transactions that gave rise to the dispute, and it bars not only claims that were brought but claims that should have been.

The four classic elements

Federal claim preclusion (the modern term most courts prefer to "res judicata," though the older label is still common) requires four elements:

1. A final judgment on the merits in the first action.
2. A court of competent jurisdiction entered that judgment.
3. The same parties or their privies were involved in both actions.
4. The same claim or cause of action is being asserted in the second case.

Each element does real work, and each one is litigated in practice.

Final judgment on the merits

A "final" judgment is one that resolves the case and from which an appeal would lie. Default judgments, summary judgments, judgments after trial, and dismissals with prejudice all qualify. Dismissals without prejudice and dismissals on jurisdictional or venue grounds generally do not, because those are not adjudications "on the merits."

The Supreme Court tightened this rule in Semtek International Inc. v. Lockheed Martin Corp., 531 U.S. 497 (2001). Semtek held that a federal-court dismissal for failure to state a claim, even though styled as "on the merits" under Rule 41(b), does not necessarily preclude a subsequent state-court action; the preclusive effect of a federal judgment in a diversity case is governed by the law of the state in which the federal court sits. Id. at 508-09. Defendants importing a federal dismissal into a state-court res judicata argument should check Semtek before assuming the prior judgment carries preclusive weight.

Competent jurisdiction

The first court must have had subject-matter jurisdiction over the dispute. This element rarely produces close cases, but it occasionally matters when the first action was filed in a court of limited jurisdiction (a small-claims court, for example) and the second action seeks relief beyond what the first court could have granted.

Same parties or privies

The simplest version of this element requires identity of parties. The more interesting version involves privity, which extends preclusion to nonparties who share a sufficiently close legal relationship with a party. The Supreme Court's modern statement of the privity rules is Taylor v. Sturgell, 553 U.S. 880 (2008). Taylor rejected the broad "virtual representation" doctrine some lower courts had adopted and instead identified six discrete categories of nonparty preclusion: (1) agreement to be bound, (2) substantive legal relationships (such as bailor-bailee or successor-in-interest), (3) adequate representation by a party (as in class actions and probate proceedings), (4) assumption of control over the prior litigation, (5) proxy litigation (relitigating through a designated agent), and (6) special statutory schemes. Id. at 893-95.

For defense lawyers, Taylor is both a tool and a warning. It allows preclusion against nonparties who fit one of the recognized categories. It also forecloses sweeping claims of virtual representation that lower courts had once tolerated. The brief should match the facts to a specific Taylor category.

Same claim or cause of action

This is the element where most res judicata motions are won or lost. Federal courts and most state courts apply some version of the transactional test from the Restatement (Second) of Judgments § 24, which asks whether the two claims arise from the "same transaction, or series of connected transactions." The transactional test is broad. Two claims are the same for preclusion purposes when they share a common nucleus of operative facts, even if they involve different legal theories, different remedies, or different evidence.

The Supreme Court endorsed a strong version of this rule in Federated Department Stores, Inc. v. Moitie, 452 U.S. 394 (1981). Moitie held that res judicata applied to bar a refiled antitrust suit even though intervening case law had vindicated the plaintiffs' legal theory. The Court rejected the argument that "simple justice" or "public policy" could create equitable exceptions to claim preclusion: a final judgment on the merits ends the matter even if it later turns out to have been wrong. Id. at 401.

That principle is harsh, and lawyers should not soft-pedal it in briefs. Moitie is the answer to the plaintiff who argues that they did not actually litigate the theory they are now raising, or that the first judgment was unjust. The point of preclusion is finality. The doctrine assumes the first court might have been wrong and treats the judgment as final anyway.

Federal-state preclusion and the Full Faith and Credit Act

A common scenario is a plaintiff who loses in state court, then refiles in federal court under § 1983 or another federal statute. Two Supreme Court decisions control.

Allen v. McCurry, 449 U.S. 90 (1980), held that issue preclusion (collateral estoppel) bars a § 1983 plaintiff from relitigating issues decided against him in a prior state criminal proceeding. The Court grounded the holding in the Full Faith and Credit Act, 28 U.S.C. § 1738, which requires federal courts to give state-court judgments the same preclusive effect they would have in the state's own courts. Id. at 96.

Migra v. Warren City School District Board of Education, 465 U.S. 75 (1984), extended Allen to claim preclusion. Migra held that a § 1983 plaintiff who had previously litigated her contract claim in state court could not later bring § 1983 claims in federal court arising out of the same transaction, because state preclusion law would have barred a second state-court suit. Id. at 84-85. The takeaway is that defendants moving to dismiss federal claims on res judicata grounds must apply the preclusion law of the state where the first judgment was entered, not federal preclusion law.

The transactional test in practice

The most common mistake plaintiffs make after a loss is assuming that a new legal theory creates a new claim. It usually does not. The transactional test treats theories, remedies, and evidence as variations within a single claim if they all arise from the same operative facts.

Examples that recur in motion practice:

• A plaintiff loses a breach-of-contract suit, then refiles as fraud based on the same misrepresentations. Barred.
• A plaintiff loses a § 1983 excessive-force suit, then refiles as assault and battery in state court. Barred where the state applies the transactional test.
• A plaintiff loses a wrongful-termination suit, then refiles for retaliation arising from the same firing. Barred unless the retaliation involves post-judgment conduct.
• A plaintiff loses a foreclosure defense, then refiles to challenge the underlying loan. Barred in most jurisdictions that treat foreclosure as fully litigating the loan's validity.

The brief should walk through the operative facts and show that they form a single transaction. Mechanical "same elements" arguments tend to lose because they reflect the older "same evidence" test rather than the transactional approach.

Compulsory counterclaims as a preclusion trap

Federal Rule of Civil Procedure 13(a) requires a party to assert as a counterclaim any claim that arises out of the same transaction or occurrence as the opposing party's claim. A litigant who fails to assert a compulsory counterclaim and later tries to sue on it standalone usually has the claim barred under what some courts call "common-law compulsory counterclaim preclusion" and others treat as ordinary res judicata.

For defendants whose adversary failed to assert a counterclaim in a prior suit, this is a powerful tool. The brief should attach the prior pleadings (judicially noticeable) and show the relationship between the prior claim and the now-asserted claim. The transactional analysis under Rule 13(a) tracks the analysis under § 24 of the Restatement.

How to brief the motion

A res judicata motion can be filed under Rule 12(b)(6) when the prior judgment is properly before the court (typically through attached or judicially noticed records). It can also be raised on a Rule 12(c) motion for judgment on the pleadings or as an affirmative defense leading to early summary judgment.

Introduction

State the prior judgment by case name, court, and date. State the present claims briefly. Then state the relationship: "Both actions arise from the same series of transactions, namely the 2019 sale of the property. The state court entered a final judgment on the merits in the prior action. Res judicata bars Plaintiff from relitigating these claims."

Statement of facts

Walk through the prior action chronologically: who sued whom, what claims were raised, what was decided, and when. Attach the complaint, key motions, and the judgment from the prior case as exhibits the court can judicially notice.

Legal standard

Two paragraphs. The first should state the four elements of claim preclusion in the controlling jurisdiction (federal courts apply federal preclusion law for federal-court judgments; state preclusion law for state-court judgments under Allen and Migra). The second should state the transactional test from the Restatement § 24 and cite Moitie for the proposition that preclusion is not subject to equitable exceptions.

Argument

Organize by element. Walk through each of the four elements, showing how the prior action satisfies it. Most space should go to element four (same claim), because that is where opposition usually focuses. End with a brief paragraph addressing any anticipated counterarguments (exceptions, lack of full and fair opportunity, intervening change in law) and citing Moitie to dispose of equitable theories.

Conclusion

Ask for dismissal with prejudice. Res judicata dismissals are always with prejudice, because the second action is precluded as a matter of law and amendment cannot cure it.

What courts get wrong and how to fix it

Three errors recur in res judicata motion practice:

Confusing claim preclusion with issue preclusion. The two doctrines have different elements and operate on different objects. Claim preclusion bars the relitigation of entire claims, including theories that could have been raised but were not. Issue preclusion bars relitigation only of specific issues actually decided. A brief that conflates them invites a court to apply the wrong test.

Treating "on the merits" too broadly. Not every dismissal is on the merits, and Rule 41(b)'s "on the merits" language does not always carry preclusive weight outside its issuing court. Semtek is the controlling authority here, and lawyers who skip it risk overstating what the prior judgment did.

Ignoring state preclusion law in federal court. When the prior judgment is from state court, federal preclusion principles do not apply. The federal court must look to state preclusion law under § 1738. A federal brief that recites federal preclusion authority for a state-court judgment will be corrected on reply, or worse, on appeal.

Bottom line

Res judicata is the cleanest dispositive motion in civil practice. It does not require the court to weigh facts. It does not require discovery. It requires a certified copy of a prior judgment, a careful application of four elements, and a brief that walks the court through the transactional analysis. The motion succeeds when the brief is precise about which jurisdiction's preclusion law applies, attaches the prior pleadings and judgment, and ties the present claims to the same transactional facts as the prior case. It fails when the defendant relies on a generic recitation of res judicata "principles" without engaging the specific transactional test, or when the prior judgment turns out to have been entered on jurisdictional or other non-merits grounds.

For plaintiffs facing such a motion, the responses are constrained but real. Argue that the prior judgment was not on the merits. Argue that the new claim involves post-judgment conduct or a separate transaction. Argue that Taylor v. Sturgell forecloses privity. What you cannot do, after Moitie, is argue that the first judgment was wrong or that justice requires a fresh look. The point of res judicata is that the first answer counts, even when it should not have.