How to enter Thailand during COVID-19?

The general rule is that you need to quarantine for 14 days. You must also get a certificate of entry (COE), a negative test, medical insurance, and book your stay in a quarantine facility.

Depending on your citizenship, your intended duration of stay, and what you intend to do, you may of course also need a visa.

There are special programs for Samui and Phuket which allow some visitors who are fully vaccinated and didn't travel to high risk countries in the last 21 days to enter without quarantine or a shorter quarantine. There are restrictions on transit via Bangkok.

Theres a lot of information out there which may be outdated. Currently there are two core entry programs to enter Thailand.

Test & Go Entry If you are vaccinated, and from the list of the 63 approved countries you may enter Thailand by staying at a SHA++ hotel until you get your RT-PCR results. After you obtain negative results you may leave the hotel.

You can find a list by going to the official TAT website https://www.thailandreopening.com/shaplus, and clicking "Alternative Quarantine".

Test & Go Entry (ATK)

For arrivals after December 15th 2021 travelers from the 63 approved countries will only have to pass an ATK test. The required hotel night booking will now be optional, however the hotel will be required to provide transport, and ATK testing through the hospital partner.

This option has been postponed until further notice due to the Omicron variant.

ASQ/AQ Entry

Travelers may enter Thailand by spending a 7 day quarantine if vaccinated, or 10 days if unvaccinated at a Bangkok ASQ/AQ Hotel, Pattaya ASQ/AQ Hotel, or Phuket ASQ/AQ Hotel. When entering using an AQ you must stay in your room for the whole duration!

Thailand Sandbox Entry (vaccinated)

Travelers may enter Thailand through the Sandbox program if they are FULLY vaccinated. They will need to spend 7 nights (previously 14 nights) at a SHA+ Sandbox Hotel. The core difference is that after their first COVID test they will be able to freely travel around Phuket, this is why they call it "quarantine-free".

EDIT: The sandbox program has been expanded to Samui, Krabi, and Phang-nga.

I will continue to keep this answer updated.

Despite not looking particularly official, this is the official website of the Tourism Authority of Thailand, which I understand to be some sort of official government office: https://www.tatnews.org/

They keep this website fully updated, and, IMHO, it is relatively easy to read and understand (at least as much as it can be given the moderately-complicated schemes they are using for entry).

We monitored the situation for months ahead of arriving (a week ago now), and this site was helpful and consistently up-to-date, unlike virtually every other government website from Thailand...