If you are a US citizen and want to get married in the Dominican Republic you will need the following documentation:

• Legalized birth certificate. You can get this document legalized at your nearest Dominican Consulate. This legalization takes 2 to 3 business days.
• Notarized and legalized Declaration of Single Status. If you are getting married for the first time you will need to write a “Declaration of Single Status” and, after notarizing it, get it legalized at your nearest Dominican Consulate.

• Legalized Divorce Degree (If you have been married before). If you have been married before, you will need to present legalized Divorce Degree. You can legalize this document at your nearest Dominican Consulate.

Note: If you are sending the documentation by mail you will need to send a prepaid envelop in order for the Consulate to send the document back.

Source: Dominican Republic Embassy, United States