What Canadian tax forms does a US-based remote employer need from me?

To properly withhold Canadian tax from your pay, a US employer generally needs to register for a Canadian payroll program account with CRA and collect the same basic information any Canadian employer needs from an employee, starting with a completed TD1 personal tax credits form so the right amount of income tax is withheld based on your personal circumstances. Without this information, the employer has no proper basis for calculating Canadian withholding correctly.

Beyond the TD1, the employer needs your relevant personal and employment details to set up payroll properly, including information needed to remit CPP and EI along with income tax, and to issue you a proper Canadian tax slip reporting your employment income and deductions at year-end, the way any Canadian employer would for its employees. A US employer unfamiliar with Canadian payroll obligations sometimes doesn't know to ask for any of this, which is part of why errors, like leaving an employee on US payroll, happen in the first place.

Because setting up compliant Canadian payroll from outside the country involves genuine administrative complexity for an employer that's never done it before, many US companies choose to work through a Canadian payroll service or an employer-of-record rather than trying to manage every requirement directly themselves.