According to MANA, the boat was unlit and hit a cargo ship on the river in the city of Pathein at about 7:20 p.m. (1250 GMT), they said.
Twenty bodies have been found and 27 people have been rescued, Kyaw Myint, a local government official, told Reuters from a hospital in Pathein.
Rescuers were still looking for survivors, witnesses said.
"The authorities at the port are looking into the cause of the crash," Kyaw Myint told Reuters by telephone. The regional government will help arrange funerals, he said.