The mishap occurred early this morning near Sanitoli village on Ghat-Nandprayag road when the bus was on its way to Nandprayag, District Disaster Management Officer Nand Kishore Joshi said.
Two persons were killed on the spot whereas 21 injured were rescued.
They have been admitted to government hospitals in Karnaprayag and Gopeshwar from where four of them have been referred to higher centres, he said.
Senior officials have rushed to the scene to ascertain the cause.