Authorities declared a disaster alert in the northern region after two days of heavy rain hit villages across the Kashmir valley, causing the worst flooding in two decades.
"At least 40 members of a marriage party are feared dead when their bus was washed away due to flash floods," senior police superintendent Mubassir Latifi told Reuters.
The bus accident occurred in Rajouri, 170 km (110 miles) south of the region's main city of Srinagar. Latifi, who was monitoring the rescue operation, said five passengers had swum to safety.
"The administration has a rescue team and helicopters ready to evacuate villagers and move them to safer ground, but heavy rain is making the rescue process very difficult," he said.
Heavy rain is expected for the next 48 hours.
The state's main river, the Jhelum, and its tributaries are flowing several feet above the danger mark. The river flows into the Pakistani side of the disputed region.
At least 15 people, including a border guard, were killed in separate rain-related incidents.
All schools and colleges were closed and exams postponed.
Home Minister Rajnath Singh held a meeting in New Delhi to review the flood situation and has dispatched members of the National Disaster Response Force to assist the state government.
Singh had been due to visit the Himalayan region on Friday but had to cancel his trip due to the bad weather.
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