The tablets were found on board a ship intercepted by the navy near the town of Kawthoung at the country's southern border with Thailand last week.
Police Brigadier General Kyaw Win, of the home affairs ministry's drug-control committee, said the haul was believed to be the largest ever made in Myanmar, which is a major producer of both heroin and methamphetamine. Similar drug busts are routinely made in Thailand, illustrating Myanmar's lack of success in combatting drug trafficking.
"We are surprised that the number of tablets seized is that big," he said, adding that the vessel also contained around 90 tonnes of timber thought to be illicit.
He said each tablet was estimated to be worth up to 80,000 Myanmar kyats ($82) on the streets of Yangon - which would make the total value nearly $200 million - but that the haul was likely destined for other countries where the price could be different.
"It's important because if it wasn't seized, it will get to Malaysia, then to US and other countries. It's expensive and the amount is also high. It's dangerous wherever it reaches," Gen Win said.
Fast-changing Myanmar is the world's second-largest opium producer after Afghanistan and Southeast Asia's biggest synthetic-drug maker, according to the United Nations Office on Drugs and Crime, which has warned that criminal activity threatens the nation's stability.
Illicit drug production in the country's war-torn borderlands has surged in recent years, particularly the manufacture of methamphetamine tablets in hidden jungle laboratories.
To mark World Drugs Day in June, Myanmar burnt seized drugs worth around $130 million, including some 1.3 tonnes of opium, 225 kilogrammes of heroin and 1.2 tonnes of methamphetamine tablets.