Foreigners entering Thailand with a visa exemption (visa-free) can change it to a Marriage Visa inside Thailand (no need to leave the country).
However, this process is called a “change of visa type,” not a new visa application.

The problem is — if you don’t have a Thai bank account, you can’t change to a Marriage Visa,
because you must show proof of a Thai bank account with at least 400,000 THB.

And most Thai banks don’t allow people on a visa exemption to open an account.

✅ In short: if you already have a Thai bank account, you can change visa exemption to a Marriage Visa in Thailand without leaving Thailand.
But if you don’t, you won’t be able to do it.

Why you shouldn’t change your visa in Thailand:

The new government policy emphasizes applying for visas online before arriving in Thailand so that the Thai embassy can verify them.

Those entering Thailand with visa-free entry or tourist visas cannot open a Thai bank account; only NON-O visas are accepted.

Many people who have entered and exited Thailand more than twice will not be allowed to change their visa in Thailand. They are often told to apply for a new visa online from abroad and then re-enter Thailand.

If you’re still not do Thai marriage registration. You should do it separately: come to Thailand first to register your marriage, then return to your home country or go to neighboring country such as Laos or Vietnam to apply for a NON-O visa online (require entry stamp to apply). After that, return to Thailand to open a bank account, deposit funds, and apply for an extension of stay.

If you plan to do everything in Thailand without preparation, it’s virtually impossible to do it all in time. This includes making appointments with the embassy to obtaining a certificate of single status, legalization the original documents, translating to Thai, certifying translations, and district office for registration, and frequent entry and exit, plus the risk of not being allowed to change your visa by immigration.