Does the Canada-US totalization agreement affect which country I pay CPP or Social Security to?

Yes. Canada and the US have a bilateral social security agreement, often called a totalization agreement, designed to prevent someone from having to contribute to both countries' systems for the same work, and to help people qualify for benefits by combining contribution periods earned in each country if they wouldn't otherwise have enough credits in either one alone. For remote workers with cross-border employment arrangements, this agreement is directly relevant to figuring out whether CPP, US Social Security, or both, should apply to their situation.

Generally, the agreement assigns coverage based on where the work is actually performed, so someone working entirely from Ontario for a US employer would typically be covered under CPP rather than US Social Security for that work, avoiding double contributions on the same earnings. Genuinely temporary cross-border assignments can work differently, which is addressed through the certificate-of-coverage mechanism discussed separately.

Because payroll systems don't always default to the correct country's contribution rules automatically, especially when a US employer is used to withholding US Social Security taxes, confirming which system actually applies to your specific working arrangement is worth doing early, rather than discovering a mismatch after contributions have already been made incorrectly.