The city and police department websites went dark around 11 a.m. Users who tried to get onto those websites got error messages instead.
The outage lasted for about an hour and a half.
The group known as Anonymous claimed responsibility on Twitter for the attack, saying, "City of Chicago govt AND Police Dept taken offline."
Anonymous is an activist group known for hacking into websites and databases. The Guy Fawkes "V for Vendetta" mask is its signature.
The group also posted video online encouraging cyberattacks against the city and stating its opposition to the tactics used by Chicago police.