The 30-year-old reacted by throwing her microphone at the concertgoer from the stage.
Although the rapper no longer had a microphone, her voice was still heard on the pre-recorded backing track playing through the speakers.
BBC News has asked representatives for Cardi B for comment.
The incident took place in Las Vegas and was caught on camera from multiple angles by several fans.
In the footage, security staff are seen surrounding the perpetrator and recovering the rapper's microphone.
However, footage from earlier in the concert showed Cardi B asking a different member of the audience to spray her from behind, as she turned her back to the crowd.
Later in the show, the rapper explained the earlier spray had been done on her back and under her own instruction, whereas she objected to the other member of the crowd unexpectedly throwing water which hit her face.
The Las Vegas Metropolitan Police Department later confirmed that a show attendee had come forward to "report a battery."
"According to the victim, she was attending an event on July 29, 2023, at a property located in the 3500 block of Las Vegas Boulevard," the Las Vegas police said in a statement. "During a concert, she was struck by an item that was thrown from the stage."
Although Cardi B's microphone was live, the fact that her voice was still heard on the backing track after she threw the mic prompted accusations of lip synching, as many jokingly compared her with the 1980s group Milli Vanilli.