Anil Kumar, 23, was having an affair with the minor girl and was posted with Hari Nagar police station, they said.
A native of Faridabad in Haryana, Kumar was living in a rented accommodation in Kondli area and called the minor girl there.
The duo had an argument this afternoon following which the girl brought over a knife from the kitchen and stabbed him on his chest.
Kumar was rushed to Lal Bahadur Shastri Hospital where he was declared brought dead.
The minor has been apprehended from the spot, they said.
