King Felipe addressed the nation in a television broadcast on Wednesday night.
Normally it is only at Christmas that there is a King's speech. The last exception was during Catalan independence troubles in 2017.
In his speech, King Felipe asked the Spain to put "differences" to one side.
"We should unite behind each other with one aim: to overcome this grave situation," he said. The king also paid tribute to workers in health care and security services.
"You are the vanguard of Spain in the fight against this illness, you are our first line of defence. You are an unforgettable example," he said about people caring for patients.