The incident comes as the former Spice Girl is in the middle of a divorce from estranged husband Stephen Belafonte.
Cowell, 57, made a poorly timed jibe after a magician suffered a technical malfunction during his performance.
“I kind of imagine this would be like Mel B’s wedding night,” Cowell said. “A lot of anticipation, not much promise or delivery.”
Brown did not appreciate the comment and proceeded to fling her glass of water at Cowell before walking out of the studio live on air.
“Mel B is out,” host Tyra Banks commented. “This is live TV, y’all. Live TV.”
Speaking to Entertainment Tonight after the moment, Brown said of Cowell, “He should know, if you say something inappropriate he’s gonna get like, stuff thrown at him.”
“In a friendly way,” Brown added to the outlet. “I made sure there was no ice in it this time… just fizzy water.”
In between takes on set, Brown said she told Cowell “he was an assh—,” to which he responded, ” ‘Oh, whatever.’ ”
PEOPLE confirmed in July that Brown was previously ordered to pay Belafonte $40,000 per month in temporary spousal support.
The singer filed for divorce in March after nearly 10 years of marriage and asked for joint legal and physical custody of their 5-year-old daughter, Madison. She also requested to terminate the court’s ability to award spousal support in her filing. The pair are due in court again in late September.