The incident took place when Arjun Ashok Tushankar (18), who worked as a house cleaner, was returning home with the girl late last night after offering prayers at a temple.
The accused, whose identity was yet to be established, accosted the youth and fired at him from point blank range in city's Savarkar Nagar locality and fled from there shortly before midnight, Vartak Nagar police sub-inspector A E Gole said.
The youth collapsed and died on the spot, he said.
The body has been sent for postmortem.
A manhunt has been launched to nab the assailant, police said, adding that it was yet to be ascertained whether the accused was alone or accompanied by someone at the time of committing the offence.
Offences have been registered under section 302 (murder) of the IPC, sections 3 and 25 of the Arms Act and sections 37(1) and 135 of the Bombay Police Act, he said.