Can I bring someone else into the lawsuit that is filed against me in Ontario?

Yes. In Ontario, if you are a defendant and you believe a third party (someone not currently in the lawsuit) is wholly or partly responsible for the plaintiff's claim — or that they owe you indemnity or contribution — you can issue a third party claim against them. This brings the third party into the proceeding so that all related issues can be resolved together.

A third party claim is a separate claim that runs alongside the main action. The third party has the right to defend both the third party claim and, in some cases, the main action. This is common in construction disputes where a general contractor is sued and seeks to involve a subcontractor, or in product liability cases where a distributor is sued and seeks to involve the manufacturer.

Third party claims must comply with the same pleading requirements as the original claim, and they must be issued within the timelines set by the Rules of Civil Procedure. If you wait too long, the court may not permit the claim or may impose conditions. Identifying potential third parties early in the litigation is an important part of defence strategy.