Is there a US–Portugal tax treaty?

Yes; the IRS publishes the treaty documents.

Do I pay social security in both countries?

The totalization agreement is designed to prevent that; which one depends on your situation.

Can I keep paying US Social Security?

Often for a temporary posting by a US employer, with a certificate of coverage. Check SSA.

How do the US–Portugal tax treaty and social security agreement work for D8 holders?

Updated 2026-11-12 · Taxes guide · Not legal advice

The US and Portugal have an income tax treaty, which helps decide where you're resident and prevents double tax, and a social security (totalization) agreement, which decides which country's social security you pay into. US citizens still file US returns and usually use the foreign earned income exclusion or foreign tax credits; a US employer sending you abroad temporarily may keep you in US Social Security with a certificate of coverage.

Which document does what
Instrument	Use for D8 holders
US–Portugal income tax treaty	Residence tie-breaker, double-tax relief
Foreign earned income exclusion / foreign tax credit	Avoid double tax on the US return
US–Portugal totalization agreement	Pay into one social security system, not both
IRS Form 6166	Proof of US tax residence (also useful for the D8's tax-residence document)

Freelancers

Self-employed people are generally covered by the social security system of the country where they live under totalization agreements. Check the SSA's agreement page and get advice before you stop paying US self-employment tax.

Quick answers

Is there a US–Portugal tax treaty?: Yes; the IRS publishes the treaty documents.
Do I pay social security in both countries?: The totalization agreement is designed to prevent that; which one depends on your situation.
Can I keep paying US Social Security?: Often for a temporary posting by a US employer, with a certificate of coverage. Check SSA.

What to do next

We don't give tax advice, but we make sure your visa file and your tax plan don't contradict each other. Start with the free D8 check; a full human review is $599.

Start the free D8 check

Sources

Checked: 2026-11-12. Rules change; confirm current values on the official pages above before you file.