Fifteen were killed on the spot and two succumbed to injuries on way to a hospital, Chamoli District Disaster Management official Nand Kishore Joshi said.
The impact of the fall was so huge, the bus was broken into pieces, a police official here said.
Going from Rishikesh to Ghat, the bus fell off a steep slope on Nandprayag Ghat motorway into a 300 m-deep gorge at about 12:45 pm, when it was around half a kilometre away from its destination, he said.
Uttarakhand chief minister Harish Rawat and Governor Aziz Qureshi expressed deep shock over the tragic mishap saying so many precious lives were lost in the accident.
Wishing the injured a speedy recovery, Rawat asked officials concerned to make all facilities available for the treatment of the injured admitted at district hospital Chamoli.
The bus was owned by Garhwal Motor Owners Company. Most of the passengers of the bus are said to have been hailing from Ghat block, the official said.