American to Marry Belgian in Belgium

You need to go to the borough hall where you will be/are living and start the process. I did it while on the three month visa that any traveler gets.
This is the lawyer I used. She is very knowledgeable and very reasonable in her fees. https://avocat-halabi.com/fr/
Some might say that you don't need a lawyer but I think it's really useful for your piece of mind.

"Some might say that you don't need a lawyer but I think it's really useful for your piece of mind." ....... and for the lawyer's bank balance.

The procedure is straightforward. You have to have:
1.Your Passport and your partners ID cart
2. A recent birth certificate for each of you (If from abroad, legalized with Apostille, translated in one of Belgium's official languages)
3.A certificate confirming you are free to marry (you are not married)

After you collect those document just go to your commune and start the process. They will advise you what are the steps after but generally expect to take some months as the police has to verify and question you that the marriage is not just to get the privilege of staying in Belgium

"Just go to the borough and start the process"

It's not so straight forward, it's easy to make a bureaucratic error that delays everything. As far as I'm concerned it's money well spent and as I said, not expensive, at least with this lawyer who is not only very knowledgeable but is a pleasure to deal with (a nice person).

Whatever path you choose good luck!

"Some might say that you don't need a lawyer but I think it's really useful for your piece of mind." ....... and for the lawyer's bank balance."

You don't tell us in the question what the nationality of your wife to be is or that you definitely want to live in Belgium, but a lawyer who is up to speed on the possible tax consequences of marrying a US citizen could well be extremely useful to your non US wife's bank balance.

@R.Harris
You mention 'very reasonable in her fees' and 'money well spent' but give no clue as to how much that might be. Your 'very reasonable' might be someone else's 'I wish I could afford that'.
@OP
When I moved here I would only have considered using a lawyer if I had found that all else had failed. And my, then, girl-friend did all the basic research for me before I moved.

@kasseietc:

I provided the lawyer's website, it is up to the person to contact her (preliminary consultation is always free). It would be irresponsible on my part to quote any fee amount, for one thing, every case is different.

as MIKEK1300GT notes above, you should very seriously consider getting proper tax and financial advice from a qualified person as there are consequences (for both you and your future spouse) that you may not have thought about. If you get yourself sorted BEFORE you leave the US / are married, it is significantly easier to manage.

"as there are consequences (for both you and your future spouse) that you may not have thought about."

Precisely.

Does your new wife realise that you may end up paying US capital gains tax on the sale of your jointly owned Belgian home, particularly if she was actually the main contributor to its purchase?

Does she understand that she will get sucked in to the US tax system and that tax system is deeply unfriendly to the point of persecution for US tax payers with foreign financial lives?

Does she realise that the financial exclusion facing Americans worldwide will now apply to her?

Of course, filing "married filing separately" still means she has to have her financial life reported in great detail to the IRS in the first year and while subsequent years mean the husband can file "married filing separately" that carries several possible negative affects that can of course ultimately negatively affect the whole family.

The list of possible problems is virtually endless and even if she accepts the cost of reporting and the negative affects of you filing "married filing separately" who pays when you miss a form and the IRS put a $10,000 penalty through your family mail box?

Oh, and don't forget that the rules for inheritance when you die are not the same for an "alien spouse" as they are for a US citizen, and these too can result in negative outcomes for your Belgian resident family.

The punitive reporting and monster penalty regime reaches out beyond the American to his business, his business partners, possibly his employer and most of all, his entire immediate family making the US citizen a financial pariah.

A direct example for Belgium of financial discrimination leading to lower income for the whole family is the example of paying an annual bonus in to a SICAV enabling the employee to withdraw it at a preferential rate to payroll.

Closed to Americans and only Americans meaning the American has to click the "no thanks" box and have his bonus swallowed by taxes.

Post FATCA US citizens are renouncing US citizenship in their thousands and once you understand how Americans with foreign finances are treated by the IRS then it becomes all too clear why they are renouncing.

Oh, and should you choose the option of renouncing in the future and your joint assets or your income exceed certain levels, the USA applies an exit tax just like good old Nazi Germany.

This could leave your Belgian wife handing 30 percent of her entirely Belgian net worth to the IRS in order for you to renounce.

Even if you don't feel you need legal and financial advice, your potential future wife does.

Trying to get a green card huh?? We learn stuff from American reality shows...90 days to marry.