The 20th stage of the three-week race around Spain took place in poor weather conditions with rain and fog affecting the climb to Alto de l'Angliru in the north-western region of Asturias.
"The young cyclist... died as a consequence of severe injuries suffered after losing control of her bike and colliding with a wall," the Asturian centre of emergency coordination said in a statement. "It appears the accident took place after the brakes failed."
The accident took place around midday local time just a few hours before Tour de France champion Chris Froome all but sealed the overall race victory.
The victim was transferred by helicopter to a hospital in the city of Oviedo, where she passed away.