I just moved here with my Thai wife from the  U.S. I have a multi entry, one year visa based on marriage. I obtained this in the U.S. If my understanding is correct, I must exit the country before the first 90 days has expired. If this is correct, what paperwork is needed? Thanks for any advice.

You only need your passport and departure card to leave the country,  then turnaround and come back using your visa to get a new 90 day entry.

You can also can extend your 90 day entry for 60 days at an immigration office to visit you wife instead of leaving when the 90 days ends.

You can also apply for one year extension on any entry.
With one year extension you only have to report to immigration every 90-days.
You need 400.000 in a Thai bank for two months or 40.000 baht monthly income.

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Thanks so much guys for your swift replies.