Can I leave Thailand with a new passport and an existing visa in my old passport?

@Olivier ******

I messaged you about the stamp in your old passport CHECK your message requests

Although I could be wrong, I am somewhat doubtful you have ANY visa in your old passport 😮

It is far more likely than not you have an 'extension of stay' from an immigration office along with a re-entry permit 🙂

If you do have an "extension of stay" stamp in your passport (w/a re-entry permit) you need to go to the immigration office that issued that extension and get the stamps moved BEFORE you travel. (that's just how the rules of extensions work)

Now on the outside chance you have a year-long, multi-entry visa issued from a thai consulate in another country (where you bounce out and back every 90 days to get a new 90 day stamp) you would indeed use both passports to exit/re-enter the country and get a new 90 day entry stamp in the new passport with a reference to the visa in an old one

Yes, just carry both passports