Do I need to do the 90-day report if I leave Thailand on the 91st day?

The rule is clear. You do 90 days report if you are in Thailand on the 90th day. How to do or try to leave the day before is another story.

Thank you,, everyone.

This is quite do-able.

By Registered Mail

Reporting by mail is slightly different than reporting in-person. You will need to send your documents in at least 15 days before your reporting due date.

You will be required to mail in signed photocopies of the following documents:

Passport info page (with photo, name, passport number, etc.)

Current visa

Latest entry stamp

Latest visa extension

Departure card (TM6) If you have one

Previous 90-day slip (if you have it)

Completed notification form (TM47) – address reporting form

These need to be submitted together with a stamp (10 Baht) addressed envelope with your home address written on the face of the envelope. The envelope is required for immigration to send back the bottom section of your TM47, which will tell you when your next reporting date is.

Would it be OK if Martin goes through passport control, thereby leaving Thailand for international air-side, before midnight? I've no idea but it was in another country.

Although you need to count your first day too. If counting that your 92 days at midnight, my idea is irrelevant.

If you are inside Thailand on the 90th day you have to do the report

As a rule, you have to do 90 days report if you are in Thailand on the 90th day. That window of 15 days before until 7 days later means you have that three weeks to report that you have been staying for 90 days as of the due date. It is based on the number of days, time wouldn't matter. You are not reporting that "I'm here in Thailand TODAY".

You may or may not be fined 2000 baht for lack of 90 days report when you go to the immigration office to apply for something. It depends on the officer if he finds it and then wants to fine you or not.