People repeat that Canada does not tax lottery wins, then forget the prize came from the US and may have US withholding or later investment income. What are the official starting points for a Canadian visitor, without giving anyone a personal tax answer?
For a Canadian resident, the two countries treat the prize separately. The CRA says US lottery or gambling winnings are not reported on the Canadian income-tax return because the prize itself is not taxable in Canada. Income earned later by investing the prize is a different matter.
On the US side, the IRS generally applies 30% withholding to US gambling winnings paid to a nonresident alien unless a treaty exemption applies; Canada is not on the current IRS list of countries whose residents receive the gambling-income exemption. The winner may need Form 1042-S and US filing advice, so a large claim deserves a cross-border tax professional.