The victim had been living with a man called Virender Singh.
The police have lodged an FIR under section 302 of the IPC. Investigation is going on.
The girl was taken to Safdarjung Hospital with serious head injuries where she was declared dead, police said.
"She was living with a man identified as Virender Singh since the last few months. His role is being ascertained," said a senior police official. The live-in partner is the main suspect.
Singh has been absconding and the police are trying to trace him.
A case has been registered at Vasant Vihar Police Station and further investigations are on.