Police official Mahesh Srivastava said the girl told her grandmother she was raped by a hospital attendant and four others when she was alone on Saturday night.
He said one man was in custody and police were looking for others.
Detectives have also asked for CCTV footage and are trying to determine why the girl was left alone in the ICU.
Mr Srivastava said the girl had been in the hospital for four days and was being treated for a snake bite.
The hospital is located 155 miles southwest of Lucknow, the capital of Uttar Pradesh.
India has been shaken by a series of sexual assaults in recent years, including the gang-rape and murder of a student on a moving New Delhi bus in 2012.
The attack shocked the country where widespread violence against women had long been quietly accepted.
Although the government has passed a series of laws increasing punishment for rape of an adult to 20 years in prison, sexual assaults are still being reported regularly.
Following outrage over the recent rape and killings of young girls and other attacks on children, India’s government approved the death penalty for anyone convicted of raping children under 12.