Starting at midnight on Saturday, "anyone in the region will have to wear a mask in public places whether they are outdoors or indoors and whether the safety distance can be maintained or not," an Extremadura government spokesman told CNN.
Extremadura is in the west of Spain, bordering Portugal.
The regions of Catalonia, whose capital is Barcelona, and the Balearic Islands announced similar measures earlier this week.
Wearing a mask in public places at all times, with few exceptions, has been mandatory in Catalonia since Thursday. In the Balearic Islands, a prime tourist destination in the Mediterranean, the new measure will take effect on Monday.
The Spanish government already requires the use of masks in public places when 1.5 meters, or 5 feet, of distance cannot be maintained between individuals. It was among the measures to remain in effect even after the nation's three-month state of emergency confinement was lifted on June 21.
Spain's 17 regional governments have the right to add their own safety rules that go further than the national standards.