The requirement will go into effect at 5 a.m. Saturday, according to Bowser's Thursday announcement.
The city ended its mask requirement for vaccinated people in May, and all residents in June, after having one of the most strict mask mandates in the nation.
Thursday's announcement comes after the Centers for Disease Control and Prevention updated its mask guidance this week for vaccinated people over the threat of the delta variant of the virus. It now urges people in some areas of the country with "high" or "substantial" COVID-19 transmission to mask up indoors.
The CDC had said in May that fully vaccinated people were not required to wear masks indoors.
The updates come as the Biden administration continues urging Americans to get vaccinated, with White House press secretary Jen Psaki saying this week that vaccines "work."
Some Republican leaders in the country, such as Texas Gov. Greg Abbott, have signaled they will not support another round of mask mandates.
"The time for government mask mandates is over—now is the time for personal responsibility," Abbott tweeted Tuesday. "In May, I signed an executive order prohibiting mask mandates by gov't entities. Every Texan has the right to choose whether they will wear a mask or have their children wear masks."
Florida Gov. Ron DeSantis's office also denounced the CDC's urging that all children be required to wear masks this coming school year.
"It isn't based in science. There is no indication that areas with mask mandates have performed any better than areas without mask mandates. In fact, this policy could actually backfire," DeSantis press secretary Christina Pushaw told Fox News this week.
"Mandating masks for vaccinated people erodes public trust and confidence in the effectiveness of the vaccines. To me it appears that the government wants to be perceived as 'doing something' during a seasonal infection surge, even if their policy does not necessarily make people safer."