Can a sub-subcontractor register a construction lien in Ontario?

Yes. The Construction Act's lien rights extend down the full contracting chain on a project, so a sub-subcontractor or a supplier to a subcontractor who isn't paid for services or materials they provided to the improvement can register a lien against the property, even though they have no direct contract with the owner. Lien rights under the Act don't depend on privity of contract with the property owner - they depend on having actually supplied services or materials that improved the property and not having been paid for them.

The same strict preservation and perfection deadlines apply regardless of where you sit in the contracting chain, so a sub-subcontractor needs to track the relevant triggering dates just as carefully as a general contractor would. Because holdback further down the chain can be smaller relative to the total value of a large project, and because there may be more parties competing for the same limited holdback fund, sub-subcontractors sometimes find it especially important to register promptly and consider whether a related trust claim against the specific funds they're owed adds meaningful additional protection alongside the lien itself.