How can I acquire Thai citizenship as the child of a Thai citizen born abroad, especially with both parents deceased?

Can’t get any Thai citizenship because you’re over 18years old.

May I ask you. Do you have Thai birth certificate?

Do you have any close relatives in thailand? I had a friend, whose parents both passed away. They did a DNA tests with her Aunt (dad's sister) and 100% match and she got her citizenship that way.

Department of Births Deaths and Marriages in US and Thailand.

If you want to obtain Thai citizenship, then your father's side is not relevant. It's even easier if they weren't married. I hope you know where your mum's birth of residence, e.g. Tambol, Amphur, or province. Go to your mum's birth registration office and request for a copy of her birth certificate. From the look of your photo, your mum shouldn't be too old so the certificate shouldn't be too difficult to obtain. Once you've got that piece of vital evidence together with your UK birth certificate (which of course contain your birth parents' names), you can ask them to register yourself and a Thai national. Things will get problematic if she'd changed her surname to your dad's. Then you will need more pieces of paper.

Come to Thailand and apply with your documents. Put cash inside the documents, it's how things work here.

Apart from Thai citizenship group you maybe able to get an ancestry visa, which allows long term stay without any financial requirements

Thailand ancestry visas | Thai Citizenship

The most important thing you can do now is to Locate ANY available Thai identification documents of your Thai Mother which you might still have possession of, which, hopefully, would include: Thai ID Card, Thai Passport, Blue Thai Family Household Book/Blue "Tabien Baan" ( ทร.14 ), perhaps her Thai Birth Certificate ( สูติบัตร )

Now, locate documents that would Link the spelling and/or changes of her Full Name from her latest Thai Passport to how your Mother's Full Name is Listed on Your British Birth Certificate, then Link the spelling and/or changes of her Full Name to how her name is shown on her American County/State Death Certificate

any Thai Marriage Certificate ( คร.3 ) and Thai Marriage Registration ( คร.2 ), along with Thai Birth Certificates ( สูติบัตร ) for those born inside Thailand should be available at the local Thai District Office (Name within Bangkok: สำนักงานเขต / Name other Provinces: ที่ว่าการอำเภอ )

I have helped an American Thai whose situation was similar to your , got some information.

-Are you in touched with your mother’s relative? You will need them as a witness when obtaining your Thai citizenship.

-Do you have Thai birth certificate? There’s a 13 digit number on it , with that your journey of obtaining the Thai citizenship will be must smoother and easier.

-How old are you? One of the downsides about being a Thai man which you are trying to be; legally, is that the soldiers duty which is not fully pardoned until thd age of 55. Of course you are able to use the facf that you’re not speaking the language nor living in Thailand is in your favor but you will need to deal with the paperwork.

PM me with any questions you may have.

You should post this in the Thai citizenship group where many people who have been through this process can help you

I was married in Thailand so the marriage was registered at some place in Bangkok but to divorce, I was told I simply could go to any amphur, so records must now be digitised and centralised, though I do not know how far back that goes.

Morning

Can you contact the Thai Department of vital statistics get information of your mother?

Where were they married ? If Thailand, was the marriage registered in the local district office ( Amphur)