Both Portugal and the United States allow their citizens to hold dual citizenship. If you are a U.S. citizen and wish to gain dual citizenship with Portugal, you must first choose what channel you will go through to get it. You cannot just "choose" to be a citizen of another country; you must be legally qualified to do so. These qualifications include eligibility for citizenship through family, marriage, or a fulfilled length of legal residency. Holding a dual citizenship with Portugal is beneficial, for it also gives you citizenship in the European Union.
Acquire citizenship through birth. Unlike U.S. citizenship law, being born on Portuguese soil does not immediately imply Portuguese citizenship. Those born to non-Portuguese citizens in Portugal are not automatic citizens unless the parents have been living in Portugal for a minimum of 10 years. If the foreign parent is from a Portuguese-speaking country, such as Brazil, the minimum number of years lived in Portugal is reduced to 6. Children born to a Portuguese citizen parent are automatically granted citizenship of Portugal, even when born on foreign soil. The procedure for claiming Portuguese citizenship through a parent must be done through the Portuguese foreign embassy in the country in which the child was born.
Acquire citizenship through naturalization. As with most countries, citizenship can be earned through a minimum length of legal residency. Legal residency in this case refers to those who live in Portugal on a legal visa of immigration (such as those with work or refugee status). The minimum length of residency required before naturalization can occur is 10 years. For those who immigrate from Portuguese-speaking countries (such as Brazil), the minimum length of residency before naturalization is only 6 years. The requirements for naturalization include passing minimum requirements for knowledge of Portuguese civics and the language. Children (under the age of 18) can only be naturalized if their parent is naturalized first.
Gain citizenship through marriage. As in most countries, one can acquire citizenship through marriage to a Portuguese citizen. There is no set residency limit for gaining citizenship in Portugal through marriage, but the couple must be married for 3 years prior to the application for citizenship. As with naturalization, language and civics knowledge are required before citizenship can be granted. All paperwork and application procedures will be done by you and your spouse inside of Portugal.