The crime happened at about 5:50 a.m. near Cicero and 5th Avenue in the Lawndale neighborhood, according to police on Saturday.
A man reportedly approached a boy and threatened him with a knife, according to police.
The suspect then forced the boy to perform a sexual act on him.
Police did not say how old the victim was.
The suspect is described to be in his 30s with locks.
Police said he was wearing all black, including a ski mask and white Nike Air Force 1 gym shoes.
Anyone with information is urged to contact Chicago police.
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