The president's son said to Breitbart News that an employee spat on him while he was visiting The Aviary, an upscale cocktail bar in the West Loop.
"It was a purely disgusting act by somebody who clearly has emotional problems," Trump told the conservative news site.
An employee at the bar also confirmed to The Washington Post that the incident took place.
An employee is accused of spitting on Trump around 8:30 p.m., WMAQ-TV reported. Secret Service agents assigned to protect Trump quickly took a woman into custody, according to Breitbart News.
Mary Ann Ahren, a reporter for WMAQ-TV, snapped a photo of authorities outside The Aviary after the incident.
Chicago police spokesman Anthony Guglielmi confirmed Tuesday night that officers were "on scene and assisting the United States Secret Service with a law enforcement matter."
Trump attributed the incident to the unidentified employee's political views.
"For a party that preaches tolerance, this once again demonstrates they have very little civility," Trump told Breitbart News. "When somebody is sick enough to resort to spitting on someone, it just emphasizes a sickness and desperation and the fact that we're winning."
Citing unidentified sources, Breitbart News reported Trump declined to press charges against the woman.