If I work remotely from Ontario for a US employer, which country taxes my salary?

If you live in Ontario, are a Canadian tax resident, and physically do your work from Ontario, Canada generally has the primary right to tax your employment income, even though your employer is based in the US and pays you from there. Canadian tax residency means you're taxed on your worldwide income, and employment income is generally sourced to where the work is actually performed, not to where the paycheque comes from or where the company is headquartered.

The US generally doesn't have the same claim on this income, because you're not physically performing services there and, typically, aren't a US citizen or green card holder either, the location of your employer's head office doesn't, by itself, give the US taxing rights over wages earned by someone working entirely from Canada. Where this can get more complicated is if you occasionally travel to the US for work, since days physically worked there can create a different picture for that portion of your income.

Because employers sometimes default to treating a remote hire as though they're still within the US tax system simply because the company is American, confirming with your employer that Canadian, not US, tax and payroll rules apply to your situation is an important early step.