Can a corporation sue a director for breaching their fiduciary duty in Ontario?

Yes. Directors of an Ontario corporation owe the corporation a fiduciary duty to act honestly and in good faith in the corporation's best interests, along with a separate duty of care to exercise the diligence and skill of a reasonably prudent person in comparable circumstances. A director who breaches the fiduciary duty - by diverting a corporate opportunity for personal benefit, acting in a clear conflict of interest without proper disclosure, or otherwise putting their own interests ahead of the corporation's - can be sued by the corporation for the resulting loss, and any improper personal gain can potentially be recovered as well.

In practice, this kind of claim is usually brought by whoever currently controls the board, since the corporation is the proper claimant for harm done to it. Where the offending director still controls the board and blocks the corporation from suing, a shareholder can potentially bring a derivative action with court leave to pursue the claim on the corporation's behalf instead. Fiduciary duty claims are fact-intensive and often turn on what the director actually knew, disclosed, and did at the relevant time, so early preservation of corporate records and communications matters.