Police officer Maung Maung Yin said the monk was stopped on Sunday as he drove in northern Rakhine state, which borders Bangladesh.
Authorities had been tipped off that the monk was carrying an illegal haul.
Maung Maung Yin said an anti-drug task force found 400,000 pills in the monk's car.
A subsequent search of his monastery turned up 4.2 million pills along with a grenade and ammunition.
A statement from the office of Myanmar's leader, State Counsellor Aung San Suu Kyi, said that one million kyats (NZD$1000) in cash was also found in the vehicle.
