The incident happened shortly after 4 p.m. at a Dollar Tree in the 700 block of Chester Pike in Prospect Park on Monday, according to police.
Authorities said a 12-year-old girl entered the store with her 6-year-old sister when the suspect appeared to follow them inside.
"This is something concerning for every parent everywhere. Obviously when it happens in a community like ours, it gets a lot of attention from us," Prospect Park Police Chief Dave Madonna said.
Police said the suspect approached the girls twice.
"He brushed by them once and then on the second occasion, actually inappropriately touched the juvenile victim," Madonna said.
After the encounter, the girl recognized the potential danger and took action, police said. Investigators said she brought her younger sister into a bathroom and called for help.
"Very poised action for a 12 year old," Madonna said. "Called family first who then called 911 and we arrived on scene almost the same time family did."
The suspect left the store before officers arrived.
Madonna said the investigation remains a public safety concern.
"This is concerning and it's something that's a public safety issue and we want to make sure that kids no matter the location are safe," he said.
Residents said the incident was unsettling and prompted conversations about stranger safety.
"Being protective of her younger sibling. It's pretty amazing. I'm in shock," said Lesley Cooper of Ridley Park.
Cooper said she now plans to speak with her own children about the dangers posed by strangers.
"I will now definitely. Definitely. You feel like it's never going to happen to you," she said.
Police said the suspect could face charges including indecent assault if he is identified and located. The investigation remains ongoing.