The incident happened about 6:30 p.m. near an alley at 101 N. Clinton St., according to police News Affairs Officer John Mirabelli.
The victim was approached by a group of seven male teens, Mirabelli said.
One, who was armed with a handgun, approached the victim and demanded his money, according to police. A second teen struck the victim on the back of his head, Mirabelli said.
The group got the boy's cell phone, cash and a bus card, police said.
Police are asking that anyone who may have seen the robbery and can provide a description of the offenders call Belmont Area detectives. Additionally, if anyone sees someone matching the description of the suspects, they should call police.
The offender with the gun was described as a black male with a dark complexion, about 19, 6-foot-1 and 170 pounds with brown eyes and black hair in dreadlocks.
The offender who struck the boy in the head was described as a black male between 15 and 18 with brown eyes and black hair, also in dreadlocks. He is about 5-foot-11 and 150 pounds, and was wearing dark blue jean shorts and a T-shirt that had the word "ENYCE" in rhinestones, Mirabellli said.
The other offenders were described only as black males believed to be in their teens.
No one was in custody as of late Wednesday morning, and Belmont Area detectives are investigating.
This incident is at least the tenth similar attack in the downtown area since the beginning of the month. Police say they have arrested more than a dozen teens but it hasn't stopped the assaults.
"We've charged pretty much everyone that's been arrested and we are going to continue to handle those cases and prosecute them fully," said Anita Alvarez, Cook County State's Attorney.
Chicago mom Gwen Bauer is just one of many no longer taking her safety for granted.
"I used to think, 'oh after dark, just don't go out at night by yourself,' but now it's happening during the day and you're scared to go out,'" said Bauer said.
"They've been attacking people in their neighborhood for a long time, but to come down here and do the same thing, it just exemplifies the fact they need to crack down on this," said Babette Peyton, Chicago resident.
Cook County Board President Toni Preckwinkle revealed Wednesday that a close friend of hers was recently attacked by what she called a mob of kids six blocks from her Hyde Park home.
"Given the world we live in, this gets more attention when it happens on Michigan Avenue than it does when it happens in our neighborhoods," said Preckwinkle.
(The Sun-Times Media Wire contributed to this report)