The truck was on its way from Lachung, North Sikkim to Gangtok when it first hit two other vehicles and then a child who was sitting on the parapet on the roadside near Chuba Khola before falling into the river at Radang near 32 No along NH-10, the police said.
The child died in the accident.
The driver of the truck told the rescuers that there were two other occupants in truck when it fell into the river.
Police inspector, Savitri Pradhan said because of heavy and continuous rains, the rescue operations have been called off for the day.
"We will start with the rescue operations in the morning," the police officer added.