The grizzly murder took place in the small town of Torremanzanas near the Spanish city of Alicante after a violent dispute between the man and his girlfriend on Monday night, Spanish media reported.
After the crime, the naked and bloody suspect was spotted wandering the streets of the town in an agitated state.
Some time later, the 37-year-old, originally from the coastal resort of Benidorm, confessed to his father who then suggested his son seek treatment at a local hospital.
Doctors then contacted Spain's Civil Guard police force and the man was arrested on Wednesday morning.
Acting on information provided by the man, they found the remains of the young woman's body buried under furniture and domestic appliances.
Preliminary investigations revealed the suspect has tried to burn the house down to hide the evidence.
News of the crime shocked the residents of Torremanzanas, a town of fewer than 800 people.
The town hall held three minutes' silence for the victim of the crime.
Neither the suspect nor his girlfriend were well known in the community, the local mayor told Spanish news agency Efe.