A 15-year-old boy was found dead inside a sleeping bag that had been taped to an overturned air mattress in an Upper East Side apartment Tuesday night, a law enforcement source said.

Takashi Tamiguchi was found by his mother in a room of the apartment on 85th Street around 9 p.m., the source said.

She had come home from work and went to get Tamiguchi for dinner, but he didn’t answer and his door was locked, the source said.

The mother was able to get into the room, where she saw the overturned mattress. When she flipped it over, she saw her son in the sleeping bag, which had been taped to the mattress, according to the source.

She cut the tape, punctured the mattress and called 911, the source said.

Tamiguchi was pronounced dead at the scene, cops said. The medical examiner will determine the cause of death.

No arrests have been made and the investigation is ongoing, police said.