Can a commercial landlord in Ontario increase rent partway through a lease term?

Not unless the lease itself allows it. Base rent for a commercial lease is fixed for the agreed term unless the lease contains a specific provision permitting a change, such as a rent escalation clause with scheduled step increases, an index-linked adjustment tied to something like inflation, or another mechanism the parties agreed to at signing. A landlord generally can't simply decide mid-term that rent is going up and demand more from the tenant without a contractual basis for doing so.

What can look like a rent increase, without actually being one, is a rise in "additional rent" — the tenant's share of property tax, insurance, and common area maintenance costs, which can legitimately fluctuate year to year based on the landlord's actual costs, separately from the fixed base rent. Tenants sometimes conflate a jump in additional rent with an unauthorized base rent increase, when the lease's cost pass-through provisions may well allow that fluctuation.

If you receive a notice of a rent change partway through your lease term, check the lease itself for an escalation or adjustment clause, and separately break out whether the increase is actually to base rent or to the additional rent components, before assuming it's improper.