The child was discovered last evening in the Thong Pha Phum district of Kanchanaburi province near the border with Myanmar, local police said.
"Officers approached him and asked him questions, but he could not speak Thai," district police chief Colonel Amnuay Pongsawat told AFP.
He said the drugs were found in about 50 plastic bags wedged into two baby powder bottles.
"It is the first time that we have seen drug trafficking gangs use a child as a courier," Amnuay said.
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The United Nations sounded the alarm this month over record seizures of methamphetamine in parts of Asia.
In its pill form -- known in Thailand as "yaba", which means "crazy medicine" -- the narcotic is used both as a party drug and pick-me-up for low paid workers with long hours.
Thai authorities netted 95.3 million pills last year -- a 93 per cent increase on 2011 -- and most of the supply was thought to have been produced in Myanmar.