The 36-year-old reportedly suffered convulsions after jumping into the pool in a residential area known as Entrenaranjos in Orihuela. Medical responders spent 45 minutes trying to save her by practising CPR. She was pronounced dead at the scene around 1.20pm yesterday, nearly an hour after the alarm was raised.
An autopsy is due to take place last night, but local reports said the unnamed woman suffered from epilepsy.
Spain is currently gripped by a severe heatwave, with temperatures rising up to 42C yesterday.
It is among a number of countries including France and Italy which is falling victim to the so-called Sarahan Bubble of hot air blasting in from Africa.
It was not immediately clear yesterday evening if she was on holiday or lived in the area.
A routine inquiry, headed by the Civil Guard force and co-ordinated by a local investigating judge, has already been launched.