If I buy a house with a tenant already living in it, do I have to honour their existing lease?

Yes. Under the Residential Tenancies Act, 2006, a residential tenancy is not ended by a change in who owns the property, so when you buy a tenanted home you step directly into the seller's position as landlord, bound by the existing lease exactly as it stood before the sale. The rent amount, the term, and any other lease conditions carry forward with the property, regardless of your own plans for it.

This surprises some buyers who assume that becoming the new owner gives them a fresh start with the tenant, but that is not how Ontario law treats the situation. You cannot simply set new rent, change the tenancy terms, or end the tenancy just because ownership changed hands; ending a tenancy still requires following the same rules that applied to the previous landlord, including proper notice and, in some circumstances, compensation. Before waiving conditions on a tenanted purchase, get the actual lease and details of the tenancy in writing from the seller, and have your lawyer review exactly what you are taking on, since the tenancy is a real, continuing legal relationship, not something the sale resets.