Local police found the 17-year-old woman, whose mouth and nose had been covered with duct tape, under a bed in his flat. She later died of her ordeal in the hospital.
The court said the man had subjected his girlfriend to prolonged and brutal violence, which had caused her extreme terror and inhumane suffering.
The verdict can be appealed.
Edit: This story was updated at 5.25pm to indicate that the convicted man was 18 years old at the time of the murder and 19 years at the time of sentencing.