Does a professional corporation protect me from being sued for malpractice in Ontario?

No. Incorporating as a professional corporation shields you from many of the ordinary risks of running a business — general contract debts, most third-party business liabilities — but it does not protect you from personal liability for your own professional negligence or malpractice. A licensed professional who incorporates remains personally liable for their own negligent acts to essentially the same extent as if they were practising as an individual or in an unincorporated partnership.

This is a deliberate feature of the professional-corporation rules, not an oversight, because the public relies on professionals carrying their own responsibility for the quality of their work regardless of how they've structured their practice. What incorporation does help with is other business exposure: leases, supplier contracts, and general operating debts of the practice can generally stop at the corporation. Professional liability insurance, not the corporate structure, is what actually protects you against a malpractice claim. If you're incorporating for tax or business reasons, it's worth having a Treadstone lawyer explain clearly what protection you are and are not getting.