Thai ATMs accept Mastercard, Visa and some even Meastro.
Cash is king and a Thai Bank will charge you 220THB per use for foreign ATM cards. Some ATMs have a maximum withdrawal of THB 20,000 some have 30,000. When you plan to use the money withdraw the highest possible amount to avoid paying the 220THB more times than necessary.

While cash is king Visa and Mastercard are widely accepted to pay with in shoppingmalls, larger supermarkets and uprange restaurants.