The mandate, which will apply to all indoor public spaces, will take effect on Monday, April 18, the city's health department said on Twitter.
Those spaces include schools, childcare settings, businesses, restaurants, and government buildings.
Philadelphia had an average of 142 cases per day and 44 hospitalizations as of April 11, a more than 50 percent increase over the previous 10 days, according to the city's health department.
The city had lifted its indoor mask mandate on Wednesday, March 2.
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