Can I sue back someone who has sued me in Ontario?

Yes, in Ontario civil proceedings, a defendant who has been sued can bring a counterclaim against the plaintiff. A counterclaim is a claim made by the defendant against the plaintiff arising out of the same or related circumstances. By filing a counterclaim within your statement of defence, you essentially become a plaintiff for the purpose of your own claim, and the original plaintiff must file a defence to your counterclaim.

Bringing a counterclaim in the same proceeding is generally more efficient than starting a separate lawsuit, because both claims can be heard together by the same judge, and the overlapping evidence is presented once. Keeping related claims together also reduces the risk of inconsistent judgments.

In Small Claims Court, the equivalent procedure is called a defendant's claim. You file a form setting out your own claim against the plaintiff or against a third party. The Small Claims Court then manages both claims together.

You can also bring third-party claims against others who may be responsible for some or all of the plaintiff's loss, drawing additional parties into the proceeding. Ontario's civil procedure rules provide for this through a third-party claim.

If you are defending a lawsuit and believe you have claims against the plaintiff, consult a lawyer about whether a counterclaim is appropriate and how to structure it.