Emergency services confirmed that they were called out to the Rocío del Mar complex shortly before 6pm on Wednesday evening after a child fell into the pool.
Paramedics attempted to revive the child but were unsuccessful and the four-year-old was pronounced dead at the scene.
The family from Britain had recently arrived at the Costa Blanca resort near Alicante in the southeastern Spain for a holiday.
Police are investigating the circumstances of the accident.
The British Embassy in Madrid confirmed the death. "Following the death of a British national in Spain, we are providing support to their family," a spokesman told The Local, adding: "Our thoughts are with them at this difficult time."
Spain records an average of 400 people drowning each year, almost a fifth of them occuring in swimming pools.