Police said another man is in custody, and charges are pending as detectives review video of the incident, which happened at Hubbard Inn in the city's River North neighborhood just after 1:15 a.m.
Authorities said a woman initially called 911 to say she had been injured.
But police say a 32-year-old man was involved in an altercation with a group, and another man, 35, tried to intervene. The 32-year-old then allegedly pushed the 35-year-old, causing him to fall down the stairs.
Police said the victim was taken to Northwestern Hospital, where he was pronounced dead. Officers took the suspect into custody on the scene.
A spokesperson for the restaurant and bar issued a statement, saying, "The Hubbard Inn team is devastated by last night's tragic incident and our hearts are with the family who lost their loved one. We can't comment on the specifics as the work of CPD is ongoing, but we are closing our doors to the public for the next couple of days while giving staff time to grieve and provide access to mental health support to those who may need it."
Hubbard Inn's manager declined an interview but shared that they are trying to support their employees and assist with the investigation.
Police are conducting a homicide investigation.
INTERACTIVE SAFETY TRACKER Track crime and safety in your neighborhood