A government decree declared the new rule would be enforced from Thursday, without specifying penalties for failing to comply.
Commuters are already obliged to wear masks on public transport in Spain, one of the hardest-hit countries with almost 28,000 deaths from the pandemic.
But the death rate has slowed and the strict lockdown measures are being gradually eased, although population centres including Madrid and Barcelona have not been allowed to relax their rules.
"Using masks will be compulsory on the street, in open spaces and any closed place of public use... when it is not possible to maintain a safe distance of at least two metres," the decree said.
It affects everyone aged six years and older, but people with breathing difficulties or other issues of necessity are exempt.
Virus transmission in Spain has slowed considerably since mid-March, when officials imposed one of the strictest lockdowns in the world.
On Tuesday, the death toll remained below 100 for the third consecutive day, down from a high of more than 900 a day in early April.