The accused, identified as Shivam Singh, allegedly molested the girl, who was known to her, at an apartment in Kashimira area of Thane's Bhayander township in the wee hours yesterday, police inspector Anil N Kadam said.
Later, when she raised an alarm, the youth allegedly slit his neck with a blade and also consumed phenol, he said.
The youth was later admitted to a hospital where he was undergoing treatment, he said.
He has been booked under sections 354 (assault or criminal force to woman with intent to outrage her modesty) and 309 (attempt to commit suicide) of the IPC, Kadam said adding that further investigation was on.