In the United States, citizenship is granted to foreigners only after a fulfilled period of legal residency of either three or five years. Immigrants who earned their permanent residency based on marriage to a U.S. citizen are eligible for citizenship after three years of residency, granted they are still married. This holds true whether the couple married inside or outside the country.
Fiance(e) Visa K-1
In order to gain U.S. citizenship through marriage, legal residency (green card) must be acquired. Foreigners who are not yet married can enter the United States on a K-1 Fiance(e) visa in order to get married and apply for permanent citizenship. The application process for K-1 begins with the U.S. citizen fiancé(e) filing form I-129F Petition for Alien Fiance(e) with United States Citizenship and Immigration Services. Following the approval of the petition, the foreign fiance(e) must apply for the visa through his nearest U.S. consulate general or embassy. Along with application forms and supplemental documents, the foreign fiancé(e) must also be able to prove proof of his relationship with the U.S. citizen. Once the visa is granted, the couple has 90 days to get married, after which the alien spouse must file for permanent residency with Form I-485.
Spouse Visa K-3
In the case of marriage to a U.S. citizen outside the United States, a marriage visa can still be applied for; however, instead of K-1 status the visa applicant must apply as K-3 status. The visa must be granted in the country where the marriage took place. Before applying, the U.S. citizens must file both form I-130 Petition for Alien Relative as well as form I-129F Petition for Alien Fiance(e). The visa interview is then conducted at the nearest U.S. consulate general or embassy to the foreign spouse. The K-3 visa allows the spouse to enter the U.S. immediately even if I-130 is still pending. After approval of I-130, the alien spouse must file for permanent residency.
Regardless of whether you entered the United States on a K-1 or K-3 visa, the residency requirement for citizenship eligibility is only three years rather than five for naturalized citizens. In order to be eligible for citizenship, the applicant must still hold her green card (that is, has committed no crimes and has not left the country unauthorized for more than a year at a time) and still be married to the U.S. citizen she earned their green card through.
After three years of residency, the alien spouse can file Form N-400 Application for Naturalization with the the USCIS. The filing fee is $675 and is concluded with an interview, English language exam and American civics exam. Once the alien spouse swears the oath of allegiance, she is given a certificate of citizenship.