Josey's attorney, Fortunato N. Perri Jr., said Josey would fight the charge. He said a full review of the evidence would reveal that Josey's actions were justified.
Perri described the events of the last month as "surreal" for Josey.
"It's been devastating for him," Perri said. "It's a crushing blow for him and his career."
Josey was recorded striking Aida Guzman, a 39-year-old Chester woman, during a chaotic street party after the city's Puerto Rican Day parade.
A group of officers had stopped a car near Fifth Street and Lehigh Avenue, Williams said, and Guzman was one of several people who circled the officers, some yelling and throwing liquid.
In the video, Guzman is seen jumping around the edge of the group, a can of what appears to be Silly String in her hand, then turning and walking away. Police said Josey and others thought Guzman threw beer at them and, as she walked off, Josey took several steps after her and struck her in the face.
Guzman was charged with disorderly conduct, but the charge was dropped. Mayor Nutter also apologized to her.
Williams said that Guzman may have acted inappropriately, and that Josey had reason to stop and question her, but not to strike.
Police Commissioner Charles H. Ramsey said he supported Williams' decision, adding that police officers are permitted to use force only in situations where they see a threat to themselves or others.
"We have a responsibility not to overreact in situations irrespective of what may be going on," he said.
Guzman's attorney, Enrique Latoison, said he was pleased with the decision, though he believed a more serious aggravated assault charge should have been filed.
Simple assault is a misdemeanor that involves one person physically striking, attempting to hurt, or threatening another, while aggravated assault is a felony charged in crimes that result or could result in serious injury.
The Fraternal Order of Police Lodge 5 has supported Josey and is appealing the decision to fire him. FOP president John McNesby this week said Williams "caved" to pressure from the media and others.
"There will be those that are going to be mad I didn't charge him with aggravated assault," Williams said Thursday in answer to that claim. "There will be those who are mad I charged him at all. But my job is not to make people happy."
Contact Allison Steele at 215-854-2641 or firstname.lastname@example.org