If your spouse was already married during the time of your nuptials, your marriage is, in all states within the U.S., considered to be legally void. This means not only can you get it annulled, but many states do not even recognize it as a valid marriage in the first place.
Annulment
Annulment is a legal process wherein a marriage is rendered null and void. Unlike divorce, in which the court ends a valid legal marriage, annulment essentially makes it as if the marriage had never existed at all. Annulment is only available to parties who fall into the list of categories that each state has for people who can void a marriage. As a general matter, situations that can result in the annulment of a marriage include:
Since every state recognizes that bigamy is not legal and that a marriage where one party was already married is void, you should have no trouble at all getting a legal annulment. Of course, it is important to note that a legal annulment is not the same thing as a religious annulment. If you want to end the marriage in the eyes of your church or another religious organization, you will have to find out the specific rules for marital annulments that pertain to that religion.
Getting Help
For assistance in filling out the paperwork and getting the marriage legally annulled, you should consult with a lawyer for guidance.