Third Party Claims in Ontario: Bringing Someone Else Into a Lawsuit

You have been sued, but the honest answer to "is this your fault" is complicated — a subcontractor did the work, a supplier provided defective material, or another driver's actions caused most of the damage. Ontario's civil process has a specific tool for exactly this situation: a third party claim ontario defendants can use to pull another party — one the plaintiff never sued — directly into the existing lawsuit.

A third party claim is the defendant's own claim against someone who was not originally part of the case, based on the argument that if the defendant is found liable to the plaintiff, that third party should have to indemnify (repay) the defendant, contribute to the loss, or is otherwise responsible for what happened. It keeps everyone connected to the same set of facts in one proceeding, rather than forcing the defendant to sue the third party separately after the fact.

When a Third Party Claim Makes Sense

• A subcontractor or supplier's work is really the cause of the problem. A general contractor sued over defective construction, for example, may believe a specific subcontractor's work is what actually failed.
• A contractual indemnity exists. Many commercial contracts include a clause requiring one party to indemnify another for certain losses — a third party claim is the way to enforce that clause within the existing lawsuit rather than a separate action.
• Shared or comparative fault is likely. Where more than one party may have contributed to a plaintiff's loss, a third party claim ensures the ultimately responsible party is part of the same proceeding.
• Insurance or professional liability is engaged. A defendant may believe an insurer, contractor, or professional adviser bears some or all of the true responsibility.

How a Third Party Claim Differs From a Counterclaim

The General Process

1. The defendant, having been served with the plaintiff's claim, identifies a party they believe should share or bear the responsibility.
2. The defendant issues a third party claim, which functions much like a statement of claim, setting out the facts and the basis for indemnity or contribution.
3. The newly added third party is served and generally gets to defend against the third party claim, much as any defendant would.
4. The third party can, in turn, raise its own defences — including disputing that it owes anything to the original defendant, or disputing the underlying facts of the plaintiff's claim itself.
5. All three (or more) parties typically proceed together through the remaining steps of the litigation — examinations, any mediation that applies, and eventually trial, unless the case resolves earlier.

Practical Considerations

• Timing matters. Bringing a third party claim early keeps the whole case moving on one track; introducing it very late can create delay and prejudice arguments similar to those raised on late amendments.
• The third party is not automatically bound by the plaintiff's original claim. It can dispute both the underlying facts of the plaintiff's case and its own responsibility to the defendant.
• A third party claim does not require the plaintiff's agreement. The defendant has the right to bring one to protect its own position, though the court retains general case-management authority over how the action proceeds.
• Construction disputes are a common setting for these claims, since projects routinely involve owners, general contractors, and multiple subcontractors and suppliers, any of whom might ultimately bear some responsibility for a defect or a payment dispute.
• Insurance defence counsel frequently rely on this mechanism where a policy or contract specifically allocates responsibility to another party in the chain.

What If You Are the One Being Added as a Third Party?

Being added to someone else's lawsuit as a third party can be jarring, especially if you had no direct dealings with the original plaintiff. You are entitled to defend the claim fully — both by disputing that you owe the defendant anything, and, where relevant, by challenging aspects of the plaintiff's underlying claim that affect your own exposure. Getting advice quickly matters, since deadlines to respond apply just as they would to an original defendant.

Frequently asked questions