The monk, named Arsara, is in custody after police discovered hundreds of thousands of the tablets in his car as he was driving from Shwe Baho village in the town of Maungdaw in Rakhine state bordering Bangladesh.
"First the police found 400,000 drug pills" when they searched his vehicle on Sunday evening, local police chief Kyaw Mya Win told AFP.
"The police then went to the monk's monastery and found another 4.2 million pills."
Myanmar is one of the world's top narcotics-producing nations, churning out huge quantities of methamphetamines as well as opium and cannabis.
The meth pills are are hugely popular across Asia among everyone from wealthy clubbers to exhausted blue-collar employees working long shifts.
Last year police confiscated a record 98 million stimulant tablets, nearly double the 50 million seized in 2015.
Drug prosecutions also jumped around 50% from 2015 to 13,500, which police said reflected the growth in the local drug trade.
Trafficking has particularly been on the rise in Rakhine state, home to more than a million people from the impoverished Muslim Rohingya minority.
In September state media reported that two men had been arrested after 6.2 million methamphetamine tablets were found in their car in Maungdaw.