Do I have to prove I suffered financial loss to win a defamation claim in Ontario?

Generally, no. For most forms of defamation in Ontario, particularly libel and the categories of slander treated the same way, damage to reputation is presumed once the basic elements of the claim are proven, meaning you do not have to separately prove you suffered actual financial loss to succeed on the claim or to receive an award of damages. This reflects the idea that reputational harm is often real and significant even where it cannot be neatly measured in dollars, and requiring specific proof of loss for every case would leave many genuine harms uncompensated.

That said, being able to show actual, provable financial or other losses - lost business, lost employment opportunities, or other documented harm - can significantly increase the damages a court ultimately awards, beyond a more general amount reflecting reputational harm alone. Some categories of slander outside the presumed-damage categories remain an exception and do require proof of actual loss, which is one reason the specific type of statement involved matters early in assessing the strength of a potential claim.