The monarch felt slight symptoms on Tuesday, with tests coming back positive on Wednesday. The king will be under quarantine for seven days and all events requiring his participation have been cancelled.
The Royal Household added that the king's general health condition is satisfactory and that he will work from his residence. Queen Letizia and the younger daughter Infanta Sofia have no COVID-19 symptoms.
The news comes on the same day as another European figurehead, Queen Margrethe II of Denmark, 81, was also revealed to have been infected with the virus.