Is US Social Security income taxed in Canada if I retire here?

Yes, but the treaty changes how much of it Canada actually taxes. Under the Canada-US tax treaty, US Social Security benefits paid to a Canadian resident are generally taxable only in Canada rather than being subject to US tax as well, which avoids the benefit being taxed twice across both countries. Within Canada, though, only a portion of the benefit, not the full amount, is included in your taxable income, reflecting a treaty-specific adjustment rather than treating it exactly like a fully taxable Canadian pension.

This differs from how CPP and Old Age Security are generally taxed, where the full benefit amount is typically included in income, so it's worth not assuming your US Social Security income will be taxed the same way as a comparable Canadian benefit. The precise portion included is worth confirming against current CRA guidance rather than assuming last year's treatment automatically carries forward.

For a retiree splitting time or having spent a career partly in the US, understanding that Social Security gets this treaty-based partial inclusion, rather than full taxation, matters for accurately estimating retirement income and planning withdrawals from other accounts alongside it.