Under the laws of most states, a United States citizen can marry an undocumented immigrant. Regardless of whether the marriage is legal, however, the marriage may not confer legality upon the undocumented spouse's immigration status.
Usually, an immigrant who marries a U.S. citizen becomes an "immediate relative" and is eligible to apply to the United States Citizenship and Immigration Service (USCIS) for a green card, i.e. lawful permanent residence. After the marriage, the U.S. citizen spouse can file Form I-130, Petition for Alien Relative and the immigrant can file Form I-485 seeking Adjustment of Status to permanent resident.
If the spouse is here illegally, however, the couple may encounter some obstacles. The spouse's illegal presence may mean that using Form I-485 to apply for permanent residence is not an option. The undocumented spouse must first leave the United States and rely on processing by a U.S. Department of State Consulate abroad before returning. Once outside the U.S, however, he or she may be barred from returning to the U.S. for years because of laws designed to punish and deter illegal immigration.
According to Section 212 of the Immigration and Nationality Act, if the spouse was present unlawfully for more than six months but less than a year, he or she would be barred from returning to the U.S. for three years. If present for more than a year, the spouse would be barred for ten years.
Under a recent change in immigration law, undocumented immigrants can apply for a provisional waiver of the three- or ten-year ban. If granted, the undocumented spouse would still have to leave the U.S. and apply at a consulate for reentry, but would not barred from returning.
Undocumented spouses must also meet the requirements that any documented spouse would have to meet. They might have to show that they are not inadmissible for other reasons, such as a criminal past, a dangerous communicable disease, or a need for public assistance. The marriage to an undocumented immigrant, like a marriage to a legal immigrant, would also have to be genuine and not a ploy to help the immigrant spouse get citizenship.
As the consequences of remaining in the country illegally can be severe, if you or your spouse is undocumented and intends to apply for citizenship based on the marriage, you should contact an immigration attorney as soon as possible.