Is it illegal to marry someone in the US to gain citizenship? What would the penalties be?

Marrying a U.S. citizen for the sole reason of gaining U.S. lawful permanent residence (a green card) is definitely illegal. If you are caught, you can face imprisonment of up to five years or fines of up to $250,000, or both. The U.S. citizen can also face criminal persecution, including fines or imprisonment, depending on the facts of the case.

Are you likely to get caught? It’s a distinct possibility. Because of concern over fraudulent marriages, the immigration laws require several steps before you can get a green card, much less U.S. citizenship.  These include:

• An initial application process that requires you to submit documents showing that you are sharing a life together, and pass an interview at which you, and in some cases your U.S. citizen spouse, must convince an immigration official that your marriage is the real thing.  If you don’t pass the first interview, you will have to undergo a fraud interview, at which you and your spouse are separately asked the same questions, and then your answers are compared.
• Following your approval for the green card, a two-year period during which your lawful residence is only “conditional,” meaning that it will expire at the end of the two years if you don’t once again convince the immigration authorities (in this case, U.S. Citizenship and Immigration Services or USCIS) that your marriage is the real thing.
• Another opportunity for USCIS to review your case if and when you apply for U.S. citizenship (naturalization). If, for example, you got divorced, USCIS might ask for further proof that your marriage was not a sham.  If you fail to provide this, not only would USCIS deny your citizenship, but it would likely put you into removal proceedings at which point you could lose the green card.

Maintaining the façade of a real marriage in the face of all these bureaucratic requirements can be difficult. In fact, sometimes the U.S. citizen simply gets tired of it all and refuses to continue with the sham.