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

American immigration law allows aliens to obtain legal residency through marriage under certain circumstances but one will not become a US citizen automatically.  Under the Immigration Marriage Fraud Amendments of 1986, aliens basing their citizenship on marriage are initially considered conditional immigrants.  Both individuals must then file a petition to remove the conditional status during the 90-day period before the second anniversary of receiving conditional status.  The INS will then interview the couple to determine if the marriage is legitimate.  If the INS determines the marriage was entered into simply for the purpose of securing immigrant status, the alien’s resident status will be terminated and the individual will be subject to deportation proceedings.

The process of obtaining legal residency in the US is an extensive process.  Although each situation is different, you should remember that before you are granted legal residency there is a two-year requirement for the marriage followed by an extensive investigation by the INS.  If at the end of this process the INS determines the marriage was only for the purposes of gaining legal residency, you will be subject to deportation.