The accident took place around 6 am on the bridge over the Kosi river in Udham Singh Nagar district's Sultanpur area, Sub-Inspector Indresh Kumar said.
The tractor-trolley, which had broken down, was hit by the truck at great speed from behind. The impact was so strong that both vehicles fell into the river by breaking the bridge's gurad rails, he said.
Three people, including a woman, died on the spot, the SI said.
The injured were admitted in a hospital in Kashipur, he said.