K1, or K-1, is a type of visa that allows the fiancé(e) of a U.S. citizen to enter the United States for the purpose of marriage. This visa is typically valid for 90 days, during which the couple must get married. After the marriage, the foreign national can apply for a green card to become a permanent resident. To qualify for a K1 visa, both partners must be legally free to marry and have met in person within the last two years. The U.S. citizen must file a petition with the U.S. Citizenship and Immigration Services (USCIS) to initiate the process.