Will I be taxed twice if I move to Portugal?

Double taxation is a real risk and a well managed one. The mechanics matter more than the reassurance.

What a treaty actually does

A convention allocates the right to tax each kind of income between the two states and sets a tie breaker when both consider you resident: permanent home first, then centre of vital interests, then habitual abode, then nationality. It does not hand you the lower of the two tax rates.

How the credit works in practice

You declare the foreign income on Anexo J of your Modelo 3 and claim a credit for the tax withheld abroad. The credit is limited to the Portuguese tax attributable to that income. So if a country withheld 15 percent on a dividend and Portugal charges 28 percent, you pay the 13 point difference here. If the foreign rate was the higher of the two, the excess is not refunded by Portugal, and reclaiming it is a matter for the other tax authority.

The parts people miss

1. Exempt is not the same as undeclared. Income that a treaty or a regime leaves untaxed in Portugal still goes on the return.
2. A treaty needs proof. A certificate of fiscal residence is what lets the paying country apply the reduced treaty rate at source rather than its full domestic one.
3. Treaties change. The new United Kingdom convention came into force on 29 December 2025 and is effective in Portugal from 1 January 2026, moving state and private pensions to Portugal alone while government service pensions stay with the United Kingdom.

Americans

Citizenship based taxation means the United States keeps taxing you wherever you live, so the filing genuinely is double even when the tax is not. That file needs a cross border specialist rather than a general Portuguese accountant.

Check your own convention on the official Portal das Financas list, and see when the Portuguese return is due.