Internal Revenue Service (U.S. Taxes)

If you are a U.S. citizen or resident alien, the rules for filing income, estate, and gift tax returns and paying estimated tax are generally the same whether you are in the United States or abroad. Your worldwide income is subject to U.S. income tax, regardless of where you reside.

When to File

If you are a U.S. citizen or resident alien residing overseas, or are in the military on duty outside the U.S., on the regular due date of your return, you are allowed an automatic 2-month extension to file your return without requesting an extension.
For a calendar year return, the automatic 2-month extension is to June 15. Note that you must pay any tax due by April 15 or interest will be charged starting from April 15.

Where to File

If you are a U.S. citizen or resident alien (including a green card holder) and you live in a foreign country, mail your U.S. tax return to:

Department of the Treasury
Internal Revenue Service Center
Austin, TX 73301-0215
USA

Useful Links
Directory of Federal Tax return preparers
Tax return preparers in Portugal

IRS contact numbers:

Nearly every tax issue can now be resolved online or by phone from the convenience of your home or office. If you need help from a Taxpayer Assistance Center (TAC), call to schedule an appointment. All TACs are now providing service by appointment.

Individuals
800-829-1040
Business
800-829-4933
TTY/TDD for people with hearing impairments
800-829-4059

For more information please see:
https://www.irs.gov/help/contact-your-local-irs-office

Useful Information

Foreign Account Tax Compliance Act (FATCA)

The Foreign Account Tax Compliance Act (FATCA) provisions of the Hiring Incentives to Restore Employment Act of 2010 (HIRE Act) were enacted on March 18, 2010.
FATCA generally requires a foreign financial institution to enter into an agreement with the Internal Revenue Service (IRS) to report information about certain accounts held by U.S. persons or foreign entities owned by U.S. persons.
For more information on FATCA, see IRS and U.S. Department of Treasury web pages.
For Joint Foreign Account Tax Compliance Act (FATCA) FAQ access here: (FATCA) FAQ

Income Tax Assistance

The IRS has discontinued the VECTA (Volunteer Embassy/Consular Tax Assistance) Program. The Embassy can no longer provide a venue for IRS-trained volunteers to assist taxpayers.
All IRS overseas operations that previously provided taxpayer assistance, including walk-in assistance, ceased effective November 20, 2015.