If you believe you are a U.S. citizen, you'll want a document to prove it.
If you were born on U.S. soil (a U.S. state or a U.S. territory, meaning Puerto Rico, the U.S. Virgin Islands, or Guam) and there is a record of your birth, in most cases, a standard U.S. birth certificate issued by a state government is your primary proof of U.S. citizenship. (Birth certificates issued by hospitals are not official records and do not serve as proof of citizenship.) If your birth was not registered within six months of your birth date, or there are other anomalies on your birth certificate, some government agencies might require the “long-form” of your birth certificate or other secondary proof of your birth in the United States.
If you were naturalized in the United States, you will have a naturalization certificate.
However, if your birth took place outside the territorial United States and you have a right to U.S. citizenship through your parents, you will not have any of these documents. (Such rights are discussed in detail in the article U.S. Citizenship by Birth or Through Parents.) In this case, you will have to apply for either a U.S. passport or a certificate of citizenship.
If you were born abroad to a U.S. citizen parent abroad and you have a claim to citizenship, you can apply for a U.S. passport in the same way as someone born in the United States. However, you will have the added requirement of establishing your citizenship claim. The evidence you'll need to have on hand might include:
Evidence might take the form of birth or citizenship records, work or tax records, school records, medical and vaccination records, travel records, baptismal records, and affidavits from you, your parents, or your grandparents.
You can apply for a passport at a passport acceptance facility in the United States or at a U.S. consulate abroad. U.S. passport acceptance facilities are designated U.S. post offices, county clerk’s offices, and other designated locations approved to accept passport applications, but the personnel at these locations are not consular officers and might not be familiar with all the documentation needed to establish your claim. Still, they should accept your application as long as you provide the basic required documentation, and they will forward it to the Department of State for a decision.
When the Department of State receives your application, it will check to see whether the documentation you provided is sufficient. If not, you will receive a letter specifying what additional documents are needed for your claim.
If you apply at a U.S. consulate abroad, you will have the opportunity to speak directly to a U.S. consular officer who has specialized training in U.S. citizenship law. The officer might be able to tell you right away whether or not you have a valid citizenship claim. Applying at a U.S. consulate is a good option if you live overseas, but if you live in the U.S. and you don’t have an existing legal way to return there, you should not rely on the possibility of receiving a U.S. passport to come back. Your claim could be denied or delayed, if you need additional documentation or have a particularly complicated claim.
If you did not acquire U.S. citizenship at birth, but have a claim to it through a parent’s naturalization, you can apply for a passport at a passport acceptance facility or apply for a certificate of citizenship.
Another way to proof of U.S. citizenship is by applying for a certificate of citizenship from U.S. Citizenship and Immigration Services (USCIS, formerly called the INS). Anyone with a claim to U.S. citizenship can apply for a certificate of citizenship. These are issued only by offices of USCIS located inside the United States.
In most cases, it is more difficult to prove citizenship through a certificate of citizenship application than by applying for a U.S. passport, mostly because it takes more time. In some of the busier USCIS offices, it typically takes over a year to obtain a certificate of citizenship. Also, the application fee is higher than for a passport. You cannot use a certificate of citizenship to travel internationally, so you might still need a U.S. passport in any case.
Evidence of your claim to U.S. citizenship should include your parents' birth certificates, marriage certificates, and naturalization certificates. You will also need your birth certificate, marriage certificate, or divorce decree to prove what your name is and to document any changes to your name. Depending on the circumstances of your claim, you might also need to provide proof that you lived in the legal and physical custody of a U.S. citizen parent.
If you were born outside the United States and at least one of your parents was a U.S. citizen at the time, they might have registered your birth with a U.S. consulate. If they did so before your 18th birthday, they would have been issued what is called a Consular Report of Birth Abroad. The consular certification is conclusive proof of U.S. citizenship.
But, if your parents did not take the steps to register your birth with the consulate before you turned eighteen, there is no way of obtaining one now. You will have to apply for a passport or certificate of citizenship using the procedures outlined above. If they did receive one, but misplaced it, you can request a replacement.
For help figuring out whether you have a claim to U.S. citizenship, see U.S. Immigration Made Easy, by Ilona Bray (Nolo).