If you don't want to break the law, and if you carry more than 20,000 US dollars into Thailand, you must inform customs. 

       In this day and age, many people choose to pay via mobile phone applications instead of holding cash. But for tourists, choosing to carry some cash sometimes makes them feel at ease, and it is convenient to spend. It is not strange for tourists to be aware of the legal requirements for bringing Thai currency or foreign currency into or out of the Kingdom of Thailand without breaking the law.  In order not to complicate it, we can divide it into two parts.

Thai Currency 

       Tourists may bring with them no more than 50,000 baht per person, but exporters to countries bordering Thailand, namely Cambodia, Laos, Myanmar, Malaysia, Vietnam, and Yunnan province of China are allowed to hold up to 2,000,000 baht per time.

       But if the amount exceeds 450,000 baht, the currency must be declared to the customs officer while passing through Customs. If a person wants to take Thai currency out of the Kingdom of more value than the legal requirement, there must be a permission from the Bank of Thailand, and the evidence of permission from the Bank of Thailand must be presented to the customs officer at the international airport when leaving the country.

       In the case of bringing money into the country, individuals and tourists may bring in Thai currency with no limit.

Foreign Currency 

       Individuals or tourists may bring foreign currency in the form of banknotes or coins out of or into the Kingdom without limits.  But if the total value exceeds USD 20,000 or its equivalent, a declaration must be made to the customs officer at the time of passing through Customs. Failure to declare items or declaring items incorrectly is a criminal offense.

Source :Royal Thai Embassy, London
Tel : +44 (0) 207 589 2944