Is truth a complete defence to a defamation claim in Ontario?

Yes. Truth, sometimes called justification, is a complete defence to a defamation claim in Ontario - if the defendant can prove the statement was substantially true, the claim fails regardless of how much harm the statement caused to the plaintiff's reputation. The defendant carries the burden of proving truth, and the statement generally needs to be substantially accurate in its overall meaning and sting, not necessarily perfectly precise in every minor detail, since courts look at whether the essential factual claim being made was true, rather than picking apart small inaccuracies that do not change the overall impression conveyed.

This defence can be harder than it sounds in practice, though, since proving truth in court means the defendant has to produce actual evidence supporting the statement, not merely have believed it was true at the time it was made. A defendant who genuinely believed something was true, but cannot actually prove it in court with real evidence, can still lose the case, even though they were not being deliberately dishonest when they made the statement in the first place.