Finality reaffirmed: the Supreme Court clarifies res judicata

In Patrick Street Holdings Ltd. v. 11368 NL Inc. (judgment released on May 8, 2026), the Supreme Court of Canada clarified the test for applying an ancient litigation principle: you do not get a second kick at the can.

The bottom line

• Duty to raise res judicata promptly: The Court reaffirmed that parties must raise res judicata at the earliest reasonable opportunity, but clarified that this requirement is not one of form, but of substance: a party must put the material facts underlying the defence in issue early, even if the doctrine is not explicitly named. That being said, it is prudent to be as explicit at possible to minimize ambiguity or uncertainty, and to ensure that parties are not barred from raising the argument at a later stage.
• Whether a cause of action is the “same” depends on the facts: Whether two proceedings involve the same cause of action depends on the underlying facts rather than the legal label attached to the claim. If the second proceeding arises from the same factual dispute, a party cannot avoid cause of action estoppel by simply advancing a different legal theory the second time around. This means that litigants should raise all viable arguments tied to the same factual dispute at the earliest opportunity, as the failure to do so may prevent those arguments from being raised in the future.
• Protection of litigants through “reasonable diligence”: Cause of action estoppel bars arguments that were made, or that could have been made, in the first proceeding, subject to reasonable diligence. Arguments that could not reasonably have been advanced earlier, including arguments based on new evidence or fraud, may not be barred by cause of action estoppel. However, the exception is limited: the party must show that the argument could not reasonably have been advanced earlier, not merely that it was not advanced.

What happened

This was a real estate financing dispute. After 11368 NL Inc. defaulted on a mortgage of $1.875 million to Patrick Street Holdings, Patrick Street enforced the mortgage by obtaining a power of sale, selling the property, and then preparing an accounting to show how the sale proceeds would be distributed. The accounting included a payment to itself for an additional $4 million under a new collateral mortgage that 11368 NL Inc. had taken shortly before the sale process. Because Patrick Street included the $4 million collateral mortgage in its accounting, the proceeds from the sale of the property were insufficient to satisfy the debts of other creditors.

Some of the other creditors successfully challenged Patrick Street’s accounting in 2016, arguing that their claims ranked in priority. The application judge excluded Patrick Street’s claim for the $4 million mortgage. The application judge reasoned that the $4 million mortgage secured up to $4 million, meaning that a lesser amount could be owing and Patrick Street had failed to prove what amount, if any, was actually owing under that mortgage. The judge ordered that other creditors be paid out instead. That decision was upheld by the Court of Appeal.

In 2019, 11368 NL Inc. applied to the court to obtain the residue of the sale proceeds. Patrick Street opposed the application, arguing that, although the $4 million collateral mortgage had been excluded from the accounting, it remained a valid contract that entitled Patrick Street to payment of the residue of the sale proceeds. 11368 NL Inc. argued that the application judge had already decided this very issue during the prior proceedings and that Patrick Street’s arguments had already been rejected. However, 11368 NL Inc. did not expressly state that they were relying on the doctrine of res judicata.

The application judge rejected Patrick Street’s arguments and ordered that the residue of the sale proceeds be paid out to other parties that were not Patrick Street.

Patrick Street appealed and advanced a new argument: the sale of the property under the power of sale was a “legal” action under the $4 million mortgage document, which constituted a default and rendered the $4 million payable to Patrick Street. 11368 NL Inc. argued that Patrick Street was barred from re-arguing their entitlement to the $4 million based on res judicata and cause of action estoppel. The Court of Appeal rejected Patrick Street’s argument on the basis that Patrick Street could not reargue its claim for entitlement to the sale proceeds under the $4 million mortgage. Patrick Street appealed to the Supreme Court of Canada.

The Supreme Court of Canada

The Supreme Court of Canada held that the Court of Appeal was correct to prevent Patrick Street from trying to recover funds under its $4 million mortgage, as its entitlement to do so had already been decided in the previous proceedings.

The key issues before the Supreme Court of Canada were (1) whether 11368 NL Inc. could argue cause of action estoppel or res judicata, given that it had not expressly raised that issue before the applications judge, and (2) if so, whether cause of action estoppel or res judicata barred Patrick Street from re-arguing its entitlement to payment under the $4 million mortgage.

Parties must address res judicata at the outset, even without formal pleading

The Supreme Court of Canada affirmed the long-standing obligation of parties to raise res judicata arguments at first instance. However, the Court clarified that parties do not have to expressly state that they are relying on this doctrine. It is sufficient to plead and argue the material facts giving rise to these arguments in substance.

11368 NL Inc. met this obligation by reproducing the 2016 decision and its resulting appeal before the applications judge and by arguing that Patrick Street’s appeal on this issue had already been dismissed. 11368 NL Inc. also appended both decisions to its pleadings before the applications judge. This was sufficient to put the doctrine of res judicata in play at first instance, even though 11368 NL Inc. had not expressly pleaded res judicata.

Clarification for assessing prongs 3 and 4 of the res judicata test

Res judicata is the overarching doctrine grounded in finality and fairness, while cause of action estoppel is the mechanism through which that doctrine operates to bar relitigation.

The Supreme Court of Canada confirmed that the circumstances of Patrick Street’s $4 million mortgage satisfied the four criteria for cause of action estoppel: (1) the prior decision was a final decision; (2) the same parties were involved; (3) the cause of action in the prior action was not separate and distinct; and (4) the cause of action of the subsequent action was argued or could have been argued in the prior action if the parties had exercised reasonable diligence.

The Supreme Court focused its analysis on the third and fourth prongs of the test.

Third prong: same cause of action

The Court clarified that, in respect of the third prong of the test, the definition of a cause of action is “a set of facts that provides the basis for an action or as a factual situation entitling a person to relief.” The Court further explained that determining whether a cause of action raised in a prior action is “separate and distinct” requires a factual assessment of the first proceeding in comparison to the subsequent proceeding, given that these facts “form the ‘cause’ of the ‘action’”. Cause of action estoppel prevents further proceedings on the same subject and parties, even if the new case uses a different legal basis or description.

In the Patrick Street case, the facts in the 2016 and 2019 proceedings were not separate and distinct. The subject matter and the relationship between the parties remained the same in both the 2016 and the 2019 actions. In both, the factual basis was about the validity, value and priority of encumbrances, including the $4 million mortgage held by Patrick Street. In other words, a comparison of the underlying facts of each proceeding led the Court to conclude that the cause of action was the same.

Fourth prong: reasonable diligence

In respect of the fourth prong of the test, the Court explained that cause of action estoppel encompasses any legal theory that was or could have been asserted in the initial proceeding through the exercise of reasonable diligence. The phrase “with reasonable diligence” serves to protect parties from circumstances where it would not have been reasonable to advance a particular theory, such as when the explanation for not initially raising the argument pertains to discovery of new evidence or involves fraud or dishonesty.

The Court concluded that Patrick Street could have raised (and ought to have raised) its basis for claiming entitlement under the $4 million mortgage in the 2016 proceedings. Patrick Street’s arguments were not based on evidence or materials that were unavailable at the 2016 proceeding but instead represented a different legal theory that it could have advanced in the 2016 proceeding. Patrick Street’s failure to advance all available arguments at the first opportunity was fatal to its position.

Dissents

Justices Karakatsanis and Martin would have allowed the appeal, emphasizing that res judicata is not applied mechanically and remains subject to a residual judicial discretion. In their view, courts retain the ability to permit a proceeding to continue where applying cause of action estoppel would work an injustice, particularly where doing so would not result in prejudice to the opposing party. They underscored that the doctrine is grounded in both finality and fairness, and that these principles must be balanced in its application. On the facts, they would have exercised this discretion to allow Patrick Street’s claim to proceed on the merits, notwithstanding the technical availability of estoppel.

Justice Côté also would have allowed the appeal, but on a different basis. She took a stricter approach to the procedural requirements governing res judicata, concluding that the doctrine had not been properly raised or adjudicated at first instance. In her view, merely placing prior decisions before the court and advancing arguments with similar effect was insufficient; the doctrine must be clearly invoked and addressed using the applicable legal test. Because res judicata was neither expressly pleaded nor decided in the 2019 proceeding, she concluded that it could not be relied upon for the first time on appeal. On this analysis, the 2019 application was decided on its merits, and Patrick Street’s claim was properly characterized as a new claim, rather than an attempt to relitigate an issue that had already been finally decided.