The boy died before emergency services arrived at a holiday complex in Torrevieja on the Costa Blanca on Wednesday, local authorities said.
It is believed he had only recently arrived at the resort with his parents.
The Foreign Office said it was providing support to the boy's family. "Our thoughts are with them at this difficult time," a spokesman added.
A four-year-old boy from the UK has drowned in a swimming pool in Spain, the Foreign Office has confirmed.