The girl was abducted in the city’s Mahim neighbourhood early Thursday while sleeping with her parents and three siblings in a pavement shack.
Mumbai Police Deputy Commissioner Vikram Deshmane her parents approached officers after realizing their daughter was missing.
Her body was found shortly afterward on the roadside nearby.
Deshmane said initial investigations suggested only one person was involved. Police are checking CCTV footage of nearby buildings.
India has been rocked by a growing number of rapes involving both children and adults in recent years.
The gang rape of a 23-year-old physiotherapy student, who died of her injuries, on a New Delhi bus in 2012 sparked national outrage.
Four men were sentenced to hang, one committed suicide in prison in 2013 and a juvenile attacker was sentenced to three years in detention.
In January 2018, the rape and murder of an 8-year-old Muslim girl, allegedly by Hindu men, stoked fury and heightened religious tensions in Jammu and Kashmir state.
She had been kidnapped while taking horses to graze and her body was found in a forest at Kathua.
The girl had been drugged, gang-raped and strangled before her body was dumped in the forest five days later, police said.
Three police officers and a former government official were among eight Hindu men arrested in the attack.
In another shocking case, a three-year-old girl was hospitalized after she was allegedly raped in New Delhi in December exactly six years after the gang-rape of the student on the bus.
Last September, a seven-year-old girl was left in critical condition in a New Delhi hospital after surgery for injuries she suffered while being raped with a water hose.
In July, an 11-year-old girl in Chennai was drugged and gang-raped by 17 men, ranging in age from their 20s to their 60s, who worked in the building where she lived, according to police.
Other recent alleged victims include a 15-year-old in Bihar state who told police in July she was repeatedly raped by a principal, two teachers and 16 boys over the course of six months.
Last April, a 17-year-old girl accused an Uttar Pradesh state legislator of taking part in her gang rape over the course of nine days in June 2017.
Last April, India introduced a temporary law making the rape of a minor or participation in a gang rape punishable by death.
According to the National Crime Records Bureau, there were 39,000 rapes in India in 2016 — roughly one every 13.5 minutes and a 12% rise over the previous year.