The additional service comes into effect on July 1, the carrier said on Monday.
The new outbound flight PG713 will depart from Suvarnabhumi Airport at 5pm and arrive at Mandalay International Airport at 6:25pm every Monday, Wednesday, Friday and Sunday.
Inbound flight PG714 will depart at 7pm and arrive in Bangkok at 9.35pm the same day.
Bangkok Airways currently uses a 162-seater Airbus A320 aircraft on the route.