Police said the victims - all women and children - were sitting on a metal slab covering the well when it collapsed under their weight.
Two more people have been injured in the accident, which took place in Kushinagar district.
Prime Minister Narendra Modi called the deaths "heart-wrenching".
The tragedy took place on Wednesday evening during the traditional "haldi" ceremony, in which relatives apply turmeric paste to the faces of the bride and groom as a marker of prosperity.
When the slab broke, other guests ran to rescue the victims and took them to a nearby hospital. While 11 people were declared dead immediately, two others died later during treatment, police said.
Uttar Pradesh chief minister Yogi Adityanath has asked district authorities to assist the families of the victims, while district magistrate S Rajalingam said they will be paid compensation.