Can a commercial tenant in Ontario negotiate a right to terminate the lease early?

Yes, and increasingly tenants do ask for this, though it has to be negotiated into the lease upfront rather than assumed or requested later once circumstances change. An early termination, or "break," right typically lets the tenant end the lease before the natural end of the term, usually after a set point, such as partway through the term, provided the tenant gives advance written notice and often pays a termination fee or penalty, which might be tied to a set number of months' rent or to recovering the unamortized portion of any allowance or incentive the landlord provided at the start.

This kind of flexibility is particularly valuable for a growing business uncertain about its space needs, or one entering a market for the first time and wanting a safety valve if things don't work out. Landlords understandably want something in return for giving up the certainty of a fixed term, which is why a termination fee or notice requirement is standard rather than a right to walk away for free.

Because this right is much harder, and often impossible, to negotiate once a lease is already signed, raise it during the initial lease negotiation if there's a real chance your space needs might change before the term ends.