The private bus, carrying 15 passengers from Siliguri to Gangtok, veered off the road and plunged into the river, reported India Today.
The accident occurred around 3:50 pm as the bus was en route to Gangtok in Sikkim.
Preliminary reports suggest that a brake failure may have led to the tragedy.
Three passengers died on the spot, while two others, critically injured, succumbed to their injuries while being transported to Rangpo Hospital.
Rest of the injured individuals have been admitted to hospitals in Rangpo and Singtam, where they are undergoing treatment.
Police, local residents, and rescue teams responded quickly to the mishap.
A JCB machine stationed near National Highway 10 for roadwork was deployed in the rescue operation. Administration is continuing to investigate the exact cause of the accident.