People who were born in the U.S. likely will have a birth certificate issued by their state government. This will provide sufficient proof of their citizenship. However, they cannot use a birth certificate from a hospital as proof of citizenship.
People who were not born in the U.S. but become citizens later will get a naturalization certificate after taking the oath of citizenship at the naturalization ceremony. The situation may be more complex for people who were not born in the U.S. but who acquired or derived citizenship through their parents. They can gain proof of citizenship by getting a certificate of citizenship or a U.S. passport.
Applying for a Certificate of Citizenship
If you believe that you have a claim to U.S. citizenship, you can pursue a certificate of citizenship from USCIS. This process will take longer than the passport process described below, sometimes lasting a year or more. The advantage of this path is that it may be easier to collect the evidence that you need to prove that you are a U.S. citizen through your parent. You will need to submit the birth certificate of your parent, their marriage certificate, and their naturalization certificate. To prove your own identity, you should provide your birth certificate in addition to any marriage certificate or divorce decree if it resulted in a name change.
Sometimes a certificate of citizenship is confused with a consular registration of birth. You may have a consular registration of birth if you were born outside the U.S. to U.S. citizen parents. They would need to have applied for the registration of birth before you turned five. This provides valid proof of citizenship. If your parents did not get a consular registration of birth before you turned five, you cannot apply for it after then. You also cannot get a replacement or duplicate of the registration of birth if the original and any copies were lost.
Applying for a U.S. Passport
Children of U.S. citizen parents also may be able to apply for a U.S. passport as proof of citizenship. They would need to show that their parents are U.S. citizens, their parents met any residency requirements that apply to their situation, and they also met any residency requirements that apply to them. (Or they can show that they were excused from meeting the residency requirements because they were unaware of them.)
A passport application usually involves a broader range of documents than an application for a certificate of citizenship. You may need to provide tax documents, records of employment, and affidavits to explain why you did not meet residency requirements because you did not know that you were a citizen. Sometimes a parent will need to submit an affidavit as well. These statements are signed and sworn under oath. In addition to those documents, you will need to provide birth certificates, proof of the citizenship of your parents, and other documents that are similar to those in the certificate of citizenship application.
You can get a passport from an office in the U.S. or from a U.S. consulate. Applications to a consulate tend to be more successful.