If a tenant is living in a property I buy at a tax sale, do they still have Residential Tenancies Act protections?

Yes - a genuine, lawful tenancy is not simply wiped out because the property changed hands through a tax sale rather than an ordinary purchase. Ontario's Residential Tenancies Act generally continues to protect a bona fide tenant regardless of how or why the ownership changed, and a new owner effectively steps into the role of landlord, along with the obligations that come with it, rather than acquiring a blank slate.

For a buyer, this means you cannot treat the tax sale itself as grounds to end the tenancy or simply ask the tenant to leave; ending a tenancy still requires following the same rules and valid grounds that would apply to any other landlord, addressed further in a related question. It also means existing tenancy terms - the rent being charged, any deposit held, and the tenant's rights around notice - generally carry forward with the property rather than resetting.

Before bidding on a property you believe is tenanted, try to learn as much as possible about the tenancy, and speak with a lawyer promptly after closing about your obligations as the new landlord.