A 24-year-old man has been arrested on suspicion of throwing his partner, 22, from the fourth floor flat they shared in Madrid.
The woman died at the scene in what police are investigating as a possible case of gender violence.
The incident happened at around 7.30am on Saturday in the Carabanchel district of the Spanish capital.
Emergency services were alerted after reports of a young woman lying unconscious in the street with a severe head injury.
Police officers arrived at the scene and attempted to assist the victim until paramedics reached the area, but they could only confirm her death.
According to police sources, investigators believe the woman was thrown from the fourth floor of a nearby apartment where the couple lived together.
Officers later located the suspect inside the property and arrested him on suspicion of homicide.
The victim was a 22-year-old Spanish woman, while the detained man is described as a Spanish national of Ecuadorian origin.
Authorities say the couple were in a relationship and the case is being treated as a potential incident of machista violence.
The woman would become the 20th killed in Spain this year in such circumstances and the 1,361st since official records began in 2003.
The investigation remains ongoing.
Spain’s domestic violence helpline, 016, is available 24 hours a day and offers support in multiple languages.
Emergency assistance is also available through 112, the National Police on 091, or the Guardia Civil on 062.
