Foreign-Sourced Income

You cannot avoid or change the jurisdiction you want to be taxed if you are a Thai tax resident, and you have overseas assets. For example, a UK pensioner cannot easily get an NT tax code while being a Thai tax resident, meaning tax will usually be deducted at the source in the UK. If you then transfer funds into Thailand, it’s taxable, but you can potentially use any tax paid as a credit against taxes owed in Thailand.

Yes, withdrawals from a Roth IRA remitted to Thailand are treated as pension income. The entire amount remitted, not just the gains, is considered taxable income.