Can I sue someone again over the same issue if I already lost a court case in Ontario?

Generally, no. Once a court has made a final decision on a matter between the same parties, Ontario law prevents those parties from relitigating the same issue or claim through the doctrines of res judicata and issue estoppel.

Res judicata (cause of action estoppel) bars a party from bringing a new lawsuit on a claim that was or could have been raised in prior proceedings between the same parties. If you had the opportunity to advance an argument and did not, you usually cannot bring a second action to raise it.

Issue estoppel is narrower: it prevents a party from contradicting a specific finding of fact or law that was necessarily decided in a prior proceeding. For example, if a court found in the first case that a contract was validly formed, that finding binds the same parties in any subsequent proceeding between them.

Courts also have a residual discretion to prevent relitigation as an abuse of process even where the technical requirements of res judicata or issue estoppel are not met — particularly where the same issue has already been fully and fairly litigated. The underlying principle is finality: once a dispute is properly adjudicated, the parties should not be able to relitigate it indefinitely.

Exceptions exist where new evidence has emerged that was not available at the first trial, but those are narrow.