The navy intercepted a ship near southern town of Kawthaun, en route to Thailand with around 2,400,000 pills of Ecstasy that police estimated was worth up to 20 million dollars.
"This seizure is the largest amount in history," senior anti-drug official Myint Oo said.
Fifteen men including the captain were arrested and detained at police station last week, he said.
Authorities in July seized heroin with an estimated value of 2.3 million dollars in the town of Tachileik in Shan state, near the Thai border.
The United Nations Office on Drugs and Crime says 10 per cent of the world's opium comes from Myanmar, the second-largest producer of the drug after Afghanistan.
Methamphet amineproduction is more difficult to trace, with the drug often produced in small, mobile laboratories in remote areas of the country, Myint Oo said.
In June, Myanmar burned around 130 million dollars worth of seized drugs, including 1.3 tons of opium, 225 kilograms of heroin and 1.2 tons of methamphetamine tablets, he said.