Police in Bhagalpur in the northern Indian state of Bihar found the eight bodies floating in the Ganges River and charged 15 people, mostly from the girl's family, with the killings.
The killings took place after Ratan Mandal, 21, eloped with Kanchan Kumari, 18, last month, police said.
"The girl's family invited the boy's family for a meeting on the pretext of settling the dispute (over the marriage) but killed all eight and beheaded them," said senior police officer Raghunath Prasad Singh said.
The whereabouts of the newly married couple is unknown, The Times of London reported.
Bhagalpur, one of India's poorest and most lawless regions, has been notorious for revenge and honor killings, The Times said.
In November a teenager who wrote a love letter to a sweetheart from a higher caste was beaten and paraded through the streets before being thrown under a train and killed as his mother begged for mercy, witnesses said.