The monk was first stopped in Rakhine state with 400,000 tablets in his car.
No value has been placed on the hauls, although in 2015 the UN estimated the retail price of a methamphetamine tablet was about $2 - making the latest consignment worth about $9.2m (£7.3m).
Myanmar has emerged as a top producer of illegal drugs in recent years.
The senior monk, named only as Arsara, was arrested after police discovered the first consignment of tablets in his car as he was driving to the town of Maungdaw bordering Bangladesh.
The second batch in the monastery was found after he was arrested.
Arsara is well known among the Buddhist community in Maungdaw, The Irrawaddy reported, and had been a leading monk at the town's Baho monastery.
Myanmar is notorious for producing vast quantities of methamphetamines, opium and cannabis, with drug trafficking a particular problem in Rakhine state.
Myanmar's illicit drug trade has flourished because of its mountainous land and porous borders.
Local reports cited by The Irrawaddy say that in late September 2016 police in Maungdaw carried out two huge drug hauls, raking in more than 15 million amphetamine tablets from the compound of a construction company.
Police in Yangon seized methamphetamine tablets with an estimated value of more than $100m (£64m) in an abandoned truck in 2015.
A UN Office on Drugs and Crime report in 2015 says that methamphetamine continues to dominate the synthetic drugs market in East and South-East Asia and is mainly available in two forms: methamphetamine tablets and crystalline methamphetamine.