If you’re a genuine visitor with a return flight they’re not likely to ask. But just in case bring your own currency to the value of 20,000 bht. That’s about £500. It’s cheaper to change cash than to use an ATM which has a high withdrawal fee. There are ATM’s but none before immigration.
**********************************************************************
%20are%20ATM%20machines%20throughout,but%20only%20at%20Citi%20ATMs.