Replacing the tenant who is leaving with a new tenant can be acceptable to a landlord and often enough the swap is permitted---though it can't be forced. In most cases your landlord cannot charge you extra fees or raise your rent because you have someone staying with you. This law sets out the rights and obligations that both landlords and tenants have. Tenants can be evicted for either they (the tenant’s guest or someone else who lives in the rental unit) do what they shouldn’t do, or they don’t do what they should. Interestingly, the landlord's consent to your current co-tenant moving out and a new roommate moving in is NOT required. Living with a Roommate.

Protection from unlawful eviction: You can be evicted for certain reasons only.

It is best if this agreement is in writing and signed by the landlord and the tenant.

Generally, tenants who rent a single or shared room have the same rights as most other residential tenants in Ontario. However, if they share a kitchen or a bathroom with the owner or the owner’s immediate family members, special rules apply. It is also your chance to sell your proposal by pointing out that your rental is big enough for another occupant and, assuming you already have someone lined up, that your new roommate will be a great cotenant. However, experience has shown that even the best of friends can become bitter enemies after several months of sharing an apartment. (1) Subject to subsections (2), (3) and (6), and with the consent of the landlord, a tenant may assign a rental unit to another person. If your name is the one on the utility bill, phone bill, or lease, then it is important that you take all necessary actions to ensure that your credit rating does not suffer in the event that your roommate fails to pay their half of the rent and other bills.

You CAN evict if: Your roommate legally subleases a room in your apartment AND you have “just cause” to evict them.
In this situation, your landlord must find “just cause,” such as breaking the lease, to evict your roommate (aka your co-tenant).

2006, c. 17, s. 95 (1). As this article describes, your roommate doesn’t have to be on the lease or rental agreement to legally qualify as a tenant or to acquire rights similar to those of an official tenant. Your local councillor or city hall might be able to help you find out what the by-law says.

One change that is particularly likely is an increase in the security deposit. Generally, tenants who rent a single or shared room have the same rights as most other residential tenants in Ontario. Boarders are not recognized by Landlords or the Ontario Tenant Protection Act. But if you live in subsidized housing where your rent is based on your income, the rules can be very different.

NOTE: Due to the COVID-19 crisis there are special temporary rental rules that apply. Both roommates and persons subletting an apartment are considered Boarders. It is common for roommates to enter into roommate agreements so that each tenant knows their rights and obligations.. A roommate agreement can include things like: Roomate Lease Agreement (Ontario) ROOMMATE AGREEMENT Suggested Information and Agreement to Help Maintain Roommate Harmony Negotiating a roommate agreement may not be your highest priority when starting out in a new place.