Can I assign my Ontario commercial lease to another business if I no longer need the space?

Generally, yes, but almost always subject to the landlord's consent, since most commercial leases in Ontario include a clause requiring the tenant to get the landlord's approval before assigning the lease to someone else. Assignment means transferring your entire interest and remaining obligations under the lease to a new tenant, who then steps into your position for the balance of the term, rather than you continuing on as the tenant in any capacity.

Whether the landlord can withhold consent, and on what grounds, depends heavily on the specific wording of your lease. Some leases require the landlord not to unreasonably withhold consent, giving the tenant a real avenue to push back on an unjustified refusal, while others give the landlord broader, even absolute, discretion to refuse for any reason or no reason at all. That difference in wording matters enormously in practice if a landlord is being difficult about a proposed assignee.

Even where an assignment is approved, many leases keep the original tenant on the hook as a backup if the new tenant defaults, unless the lease specifically releases them. Read your lease's assignment clause closely, and get advice before assuming you're free of the space once someone else takes it over.