Key takeaway: Dual citizenship depends on the law of both countries. Current German, Japanese, Irish, Italian and Portuguese rules and US tax filing are checked; the unsupported 35-country table was removed.

Both countries decide

Holding two nationalities depends on the law of the country you already belong to and of the country you are joining. Japan’s Ministry of Justice states that Japanese nationality is automatically lost when a foreign nationality is acquired by choice, for example through naturalisation. From 1 April 2022, Japanese multiple nationals must choose a nationality by age 20 if they became multiple nationals before 18, or within two years otherwise.

Ministry of Justice of Japan: nationality Q&A

Germany and Portugal: residence before naturalisation

Germany’s Nationality Act (section 10) provides for naturalisation after five years of lawful habitual residence, subject to further conditions including German at level B1; giving up a previous nationality is not among its listed conditions, and the former shortened-residence paragraph is marked as repealed. Portugal’s Organic Law 1/2026 requires at least seven years of legal residence for nationals of Portuguese-speaking countries and EU citizens, and 10 years for others. It entered into force the day after its publication on 18 May 2026; pending applications keep the previous wording.

German Nationality Act section 10 (StAG) Diário da República: Organic Law 1/2026 (Nationality Law amendment)

Citizenship by descent: Ireland and Italy

Ireland’s Foreign Birth Registration allows people born abroad to become Irish citizens through a grandparent born in Ireland, or through a parent who was an Irish citizen at the time of their birth. Italy changed its rules by Decree-Law 36 of 28 March 2025, converted by Law 74 of 23 May 2025: people born abroad who hold another citizenship are not deemed automatic citizens unless an exception applies, such as a parent or grandparent who held exclusively Italian citizenship, or a parent who lived in Italy for at least two continuous years after becoming Italian and before the applicant’s birth.

Department of Foreign Affairs: Foreign Birth Registration Italian Ministry of Foreign Affairs: citizenship

Tax follows the rules of each country

The IRS states that US citizens and resident aliens abroad are subject to US tax on worldwide income from all sources; some qualify for benefits such as the foreign earned income exclusion. A second nationality can change your filing obligations, so ask a tax adviser in each country before you acquire or use it.

IRS: U.S. citizens and resident aliens abroad

Questions before acting

Is there one list of countries that allow dual citizenship?

Not in this review. The outcome depends on both countries’ laws and their effective dates, so check each one.

Did Portugal change its naturalisation period?

Yes. Organic Law 1/2026 sets seven years for nationals of Portuguese-speaking countries and EU citizens and 10 years for others; pending applications keep the previous rules.

Corrections in this revision

The previous version included statements without sufficient direct support. This revision removes or corrects:

  • Unsourced statistics (“76%” of countries; “89 countries allow dual citizenship”) and the 35-country policy table
  • Outdated rules: Japanese choice of nationality by age 22 and Germany’s three-year fast track
  • Italy described as having no generational limit on descent, despite the 2025 change
  • Fastest-citizenship claims (Argentina two years, Paraguay three years) without official sources

Review scope: legal and financial statements in this revised article. Recheck on a provider, professional, immigration or tax-rule change; reuse of dated figures requires a new check.

See recorded sources

These primary references have the jurisdiction, product and scope stated above. A source link is not proof of personalised eligibility.

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About the author

António Mira is the founder of whereTOemigrate. For over a decade he has worked at the intersection of Portuguese real estate and overseas business, helping international clients navigate the cross-border decisions that come with relocating. He sits on the board of CCIAP (Portuguese-Arab Chamber of Commerce), the Portuguese-Saudi Business Council, and the Portugal-Hong Kong Business Association — always with one focus: enabling overseas business and human mobility into and out of Portugal.

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