If someone sues me for breach of contract, can I sue them back in the same case in Ontario?

Yes. In Ontario civil litigation, a defendant who has been sued for breach of contract can file a counterclaim against the plaintiff in the same action. The counterclaim sets out the defendant's own claim against the plaintiff and is resolved at the same trial, avoiding duplicative proceedings.

A counterclaim can be for the same subject matter — for example, if the plaintiff claims you breached the contract and you say they breached it first — or it can be a separate claim arising from a different transaction between the same parties.

In Superior Court proceedings, a counterclaim must be served and filed within a specific time after delivering a statement of defence. Failing to bring a related claim as a counterclaim can sometimes cause problems later if you try to start a separate action, because the court may consider whether the issues ought to have been raised in the original litigation (issue estoppel or abuse of process).

In Small Claims Court, a defendant can make a "defendant's claim" against the plaintiff or a third party. The procedure is somewhat simpler, and the amount in dispute stays within the Small Claims limit.

Raising a counterclaim early and strategically — not just defensively — can sometimes shift the litigation dynamic significantly.