Could I adopt a person with mental retardation and get some kind of visa on the back of it?

Not that I am aware of.

Would this person be Thai or a foreigner? Any relationship to your family?

Thai and no relation.

Couldn't I get some kind of visa on the basis of being a guardian?

Edited November 19, 201510 yr by notmyself

You should ask an international adoption agency if you are qualified to adopt a person.

No Thai visa or extension of stay rule uses the term guardian. Only parent is used. I think at times people should look up the definition of guardian ( http://www.merriam-webster.com/dictionary/guardian ).

You might be able to get a non-o visa at an embassy or consulate in your home country based upon being the parent of a adopted Thai.child.

There is only one clause of the police order that allows an extension for being the parent of a Thai. It mentions adopted but is not clear that it would be granted. I suspect most immigration office would not allow an application. See clause 2.18 of Police Order 327/2557 basis for extension of stay and Immigration Bureau Order 138/2557 documents for extension of stay.

Thai and no relation.

Couldn't I get some kind of visa on the basis of being a guardian?

No.

That would be an idea of visa for "general support of a Thai person"

If he adopts a child, does not he automatically become his legal parent?

If he adopts a child, does not he automatically become his legal parent?

Yes but only as an adopted parent which is not the same as being a parent by birth when it comes to visas, citizenship and etc.

Guess adoption offer will be off the table now as does not provide visa. Geezus

As you said "I" not "we" I assume you are single, and male? If so you are not permitted to adopt a Thai child with special needs.