Yes. If you have had an extramarital affair within the Good Moral Character period that is required in order to naturalize (usually the past five years), it is possible you might not qualify for U.S. citizenship.
Whether you qualify in spite of your extramarital affair will depend on the facts and details of what happened. While an extramarital affair does not automatically disqualify you, examiners at U.S. Citizenship and Immigration Services (USCIS) are taught to ask more detailed questions if they find evidence of an extramarital affair. If your affair led to the breakup of your marriage within the Good Moral Character period, they will likely deny your application.
As an applicant for naturalization, you must demonstrate Good Moral Character (GMC) for a specific period of time. This may be one year, three years, or five years. For most people, the GMC period is five years. Special residency rules may apply to residents married to U.S. citizens, immigrants who have served in the military, or battered spouses of U.S. citizens.
Sex outside of marriage used to be considered automatic evidence of bad moral character. That is no longer the case. But cheating on a spouse is still considered bad moral character.
What's the difference? Mostly it depends on whether you were living with your spouse when you had the affair, and whether your spouse had full knowledge of the relationship. An outside relationship when you and your spouse are separated, or an outside relationship that your spouse knows about and accepts won't be considered evidence of bad moral character.
But on the other hand, if you were still living with your spouse when you had the affair and if your spouse did not know and approve of the relationship, that will be considered cheating on a spouse, and evidence of bad moral character. This is especially true if your affair led to your divorce.
So, if your application for naturalization indicates that you have had an extramarital relationship, the USCIS officer who interviews you and reviews your case will want more details about your situation.
The most common evidence of an extramarital affair is that the applicant had one or more children born or conceived with one person while still married to someone else. This is not hard for USCIS to figure out.
You are required to list ALL your children on your N-400 application for naturalization. This includes adopted children and stepchildren, as well as all biological children. The form requires you to state the date and country of birth of each child, where each child currently lives, and whether you have ever failed to support them.
While the form does not ask you to name the other parent of each of your children, USCIS officers are trained to ask clarifying questions if the facts on your N-400 application don't appear to make sense: for instance, if you are the father of two children born less than nine months apart, or if the surname of your child is different from the surname of the man you were married to.
The second most common evidence of an extramarital affair is children born during a marriage that ended in divorce, where the divorce decree does not include or make financial support arrangements for the child. Sometimes a divorce decree will state that a child born during the marriage is not the child of both parties; other times the divorce decree will simply fail to mention the child. In either case, USCIS examiners read through divorce decrees carefully, and they will notice that the facts don't match.
Finally, you will need to prove that you are current on all child support obligations. If, for example, you have been taken to court and required to pay child support for children born to one person while you were married to someone else, that is also evidence of an extramarital affair.
Because an affair is no longer automatically considered to show bad moral character, USCIS officers are trained to ask follow-up questions. They will want to know whether you were separated from your spouse when you had the affair. They will especially want to know whether your affair caused the breakup of your marriage.
If you are still married and living with the same spouse, and the affair occurred during a period of marital difficulty that you have now worked through and resolved, USCIS will probably approve your case. If you and your spouse were already separated and in the process of divorce at the time of the affair, your application will probably be approved. If you are separated due to domestic violence by your spouse, or for some other reason you did not cause (your spouse is or was incarcerated), even if you are still married, your case may be approved.
As a general rule, if you were still living with your spouse when you had the affair, USCIS will rule that your affair contributed to your divorce. If that is the case, or if you know that your affair contributed to the breakup of your marriage, then the only way to be certain that your application will not be denied is to wait five years (or the length of your required GMC period) from the end of the affair or from the date of your divorce (whichever comes first) before applying for naturalization.