Who is responsible for maintaining a retaining wall built on the boundary line between two Ontario properties?

There's no single clean statutory rule assigning retaining wall responsibility in Ontario the way the Line Fences Act does for boundary fences, so responsibility tends to depend on the specific facts: any agreement between the owners, who actually built the wall and why, and sometimes applicable municipal requirements. Where a written agreement exists, that governs; where it doesn't, the analysis gets more fact-specific.

A key factor courts and neighbours often look at is which property the wall is actually retaining or benefiting. A retaining wall built to hold back a grade change that primarily supports one property's yard or foundation is often treated as more that property owner's responsibility, even if it happens to sit on or near the boundary line, since that owner is getting the direct structural benefit. But this isn't a fixed rule either, and the specific history of who built it and under what circumstances can shift the analysis.

Because there's no bright-line answer here, and a failing retaining wall can create real safety and drainage risks for both properties, neighbours dealing with an existing wall of uncertain responsibility should document their understanding in writing now, or get advice on the specific facts before a failure forces the issue.