Which steps do I have to take to marry my foreign partner?

Partners from a country that doesn't require a visa can travel to Belgium with a valid national pasport. Partners from other countries require a visa. (visa type C)

You find the list of necessary papes and the way to obtain the visa on the website of the department of Internal Affairs & Immigration Office.

At the Belgian embassy or the Belgian consulate situated in the country of origin you ask for a visa type C.

In the first three days after your arrival in Belgium, the non-Belgian partner must register an arrival declaration at the city hall of the place where he or she resides. The non-Belgian partner will receive a "Supplement 3" with which it is legal to stay in the country for 90 days.

the civil marriage is best performed during these 90 days. If this doesn't happen, you can ask the Immigration office to extend the document, but usually administration  doesn't do that. The marriage must be performed at the earliest 14 days after  the date of the issuing of the marriage declaration and at the latest 6 months after that date. So you can make sure you have the marriage declaration in order before the partner enters the country.

Of course you need some documents to marry. You need:

• Birth certificates for both parties
• Proof of identity
• Proof of nationality
• Certificate of residence, showing full name, place and date of birth, last legal residence, nationality and marital status for both parties. A person who is not a Belgian resident must obtain their certificate from the authorities in their last place of residence. If this is not possible (the US authorities do not issue such documents, for example) then ask the local Belgian authorities what they will accept instead
• If either party has previously been married (in a heterosexual or same-sex marriage) they must provide a copy of the final divorce decree (or death certificate of the former spouse if widowed). Any divorce decree must show the date on which the divorce became final. If not, the court that pronounced the divorce must provide a statement giving this additional information
• Normally both partners go to City Hall together to declare the marriage. If only one of the partners does this (because the other one is still in the country of origin, for instance) then you need a legal written proof that states that the absent partner agrees with the declaration.
• Act of Common Law: only the partner from another country needs to present this document. It is an authentic document from the country of origin that specifies the conditions of marriage according to the country of origin. This document allows the official of the civil registry to check if those conditions apply to the partner from that country.

Once the documents have been delivered, the relevant registrar in Belgium draws up the official copy of the marriage notice form (acte de déclaration de marriage/akte van aangifte van het huwelijk).