U.S. social security benefits paid to a resident of Canada are taxed in Canada as if they were benefits under the Canada Pension Plan, except that 15% of the amount of the benefit is exempt from Canadian tax.
According to the IRS, special tax treatment applies to payments received from the Canadian
pension, the Quebec pension plan, and the Old Age Security plan. If the recipient is a resident of the United States, the benefits are taxable only in the United States, treated as US social security benefits for US tax purposes