The trailer was carrying around 50 people, mostly women and children, when the incident happened in Kanpur in the northern state of Uttar Pradesh.
A tractor pulling a trolley packed with religious pilgrims in India overturned and plunged into a pond, killing 26 and seriously injuring 16, officials said.
The trailer was carrying around 50 people, mostly women and children, when the incident happened in Kanpur in the northern state of Uttar Pradesh on Saturday, senior local official Sudheer Kumar said.
"Twenty-six people died in the incident," Kumar said.
Traffic accidents are frequent in India with more than 150,000 people killed last year, or over 400 per day, according to official statistics which are likely an underestimate.
People being transported on tractor trailers is also common, particularly in rural areas, on often poorly maintained roads where there is scant regard for safety and traffic regulations.
Prime Minister Narendra Modi offered his condolences after the latest incident which happened as the tractor was on its way back from a Hindu temple.
"Distressed by the tractor-trolley mishap took place in Kanpur. My thoughts are with all those who have lost their near and dear ones and prayers with the injured," Modi tweeted.
Uttar Pradesh Chief Minister Yogi Adityanath said this kind of vehicle -- a tractor pulling a large cart -- should be used only to transport farm goods and freight, not people, according to The Hindu newspaper.
"The road accident in Kanpur district is very heart-wrenching," he said.