Can I sue the person who is suing me while the lawsuit is ongoing in Ontario?

Yes. In Ontario, a defendant who has a claim against the plaintiff arising out of the same or related transaction can assert it as a counterclaim within the same proceeding. Rather than starting a separate lawsuit, you add your claim against the plaintiff in your statement of defence, which becomes a statement of defence and counterclaim.

The advantage of a counterclaim is efficiency — both claims are heard together, reducing cost and avoiding inconsistent outcomes. The plaintiff then becomes a defendant to the counterclaim and must file a reply and defence to counterclaim.

You can also assert a crossclaim against another defendant in the action if you believe they share liability for the plaintiff's claim or that they are liable to you. This is common in construction and multi-party disputes. Counterclaims and crossclaims are governed by the Rules of Civil Procedure and must comply with pleading requirements. If you have a legitimate claim against someone who is already suing you, assert it early — delays can complicate the process.