Helsinki, Espoo and Vantaa are starting to require the use of face masks in the indoor sports facilities they manage.
The mask must be worn other than during exercise.
On Tuesday, the Helsinki Metropolitan Area Corona Coordination Group outlined the issue.
The obligation will take effect on November 2 and applies to all 15-year-olds.
The mask is not required for those who cannot use it due to their health condition.
The coordination group and the cities are also calling on businesses and non-profit actors to oblige them to use masks in their own indoor sports facilities.
The meeting also decided to keep public spaces in the cities open.
The Department of Health and Welfare (THL) said on Tuesday that 193 new coronavirus infections have been reported in Finland. 2,664 infections have been reported in two weeks, 57 more than in the previous two weeks.