Mukdahan customs office chief Woraphat Chaovisit said on Friday a woman identified only as Matapon, 42, had tried to pass through the Thai-Lao Friendship Bridge immigration 2 checkpoint with 30 bundles each containing one million baht in cash.
Customs officials had been tipped off and moved in when the target pickup truck arrived at the checkpoint.
The money was found under the front and back seats and in a tool box inside the vehicle. All the cash bundles had Krungsri Bank wrappings around them.
Ms Matapon said her boss in Laos transferred the money to pay for the purchase of energy drinks, but she did not make the payment because the bank had already closed. So she decided to take all the money back to Laos. She was arrested around 7pm Thursday.
Mr Woraphat said customs officers had not bought the explanation and remain suspicious.
She was informed that under Thai law a person cannot take more than 500,000 baht in cash out of the country without prior permission.