Spain's King Felipe VI started 10 days of quarantine on Monday after coming into close contact with someone who later tested positive for the new coronavirus, a Royal House source said.
The king, 52, has cancelled his public appearances during the quarantine period after the person tested positive on Monday, one day after they were in close contact, the source added.
Felipe VI had chaired a scientific meeting in Madrid earlier on Monday.
Queen Letizia and their two daughters will continue their royal activities, the source said.
Spain has registered more than 1.58 million COVID-19 cases - western Europe's second highest tally after France - and 43,131 deaths.
The overall number of infections has grown by almost 26,000 since Friday and the number of deaths has increased by 512, the Health Ministry said on Monday.
New infections measured over the past 14 days fell to 374 per 100,000 people as of Monday, from 530 in the first week of November.