The ill-fated bus carrying about 60 pilgrims, mostly Indians, was returning to Krishnanagar of Kapilvastu from Swargadwari temple when it veered off the road and plunged into Madi Khola river in Pyuthan last evening.
Police said the bus rolled about 100 meters before it plunged into the river near Lamachaur village, 400 kms east of Kathmandu.
Chief District Officer Ram Bahadur Karumbang said most of the victims were Indian pilgrims.
Nine persons were killed on the spot while two died on their way to hospital.
Six others succumbed to their injuries in hospital, police said.
Four Indian deceased have been identified as Satguru Harjan, Sanahi Harjan, Fulo Harjan and Dilu Harjan, they said, without giving further details.
More than 40 injured have been admitted to hospital.
Investigations are underway to identify others killed in the accident.