"To help continue efforts to reduce the spread of the coronavirus, beginning Monday, June 8 everyone riding SEPTA will be required to wear a face mask or covering. #FlattenTheCurve," SEPTA tweeted Friday.
The transit agency has been strongly urging customers to wear face coverings while riding, but it was not a requirement.
In April, SEPTA had issued a policy requiring riders wear masks, but reversed its decision after videos of two separate incidents were posted on social media.
SEPTA changes mask policy after videos surface on social media
One video showed a man being pulled off a bus by several Philadelphia police officers after the bus driver called 911. A police report stated the rider caused a disturbance on the bus stemming from a disagreement over wearing a face mask.
The second video circulated showed a SEPTA worker telling people on a bus to exit if they didn't have masks. The worker then directed one man, who was wearing a bandanna over his mouth, to get off the bus.
Regular schedules are in effect on most SEPTA bus routes, trolleys, the Norristown High-Speed Line, Market-Frankford and Broad Lines.