The victims, all women and children, were sitting on a metal slab covering the well when it fell beneath their weight, according to police.
Two other persons were hurt in the tragedy, which occurred in the Kushinagar district.
Prime Minister Narendra Modi described the deaths as “heartbreaking”.
The incident occurred on Wednesday evening, during a traditional “haldi” ritual in which relatives apply turmeric paste to the bride and groom’s faces as a sign of wealth.
When the slab collapsed, other guests rushed to the victim’s aid, transporting them to a nearby hospital. While 11 individuals were confirmed dead at the scene, two others died later following treatment, according to authorities.