What happens tax-wise if I withdraw from my US 401(k) after becoming a Canadian resident?

Once you're a Canadian resident, a withdrawal from your US 401(k) generally becomes taxable in Canada as income, in much the same way an RRSP or RRIF withdrawal is taxed for a Canadian resident, because Canada taxes its residents on worldwide income and the treaty-based deferral you may have relied on while the account was growing ends once money actually comes out. The US may also apply withholding tax on the same withdrawal under its own domestic rules, though the Canada-US treaty can reduce that US withholding rate for a Canadian resident compared to what a US person might otherwise face.

Because the same withdrawal can be taxed in principle by both countries, you're generally entitled to claim a foreign tax credit in Canada for the US tax properly withheld, reducing your Canadian tax on that income to avoid being taxed twice on the same dollars, up to the Canadian tax otherwise payable on it.

Getting the timing and reporting right on both sides of the border, and confirming the correct treaty withholding rate applies rather than a higher default US rate, is where advice becomes valuable, since a 401(k) withdrawal that isn't handled correctly can end up more heavily taxed than it needs to be.