The reality TV star stepped in the ring with a female fan for a charity boxing bout at the Commerce Casino in California.
Tweeting to fans before the fight, Kardashian admitted she was "sooo scared".
Her stepfather Bruce Jenner and brother Robert also laced up for charity. All three lost their bouts.
Kardashian reports she left the ring with a black eye and her brother needed medical attention for a concussion.
In a Wednesday morning (04Nov09) post on her blog, the shocked star writes, "We literally had three days of training to prepare for the event. We didn't expect what happened.
"I knew I had to do it (fight) for charity, since that's what it was all about, but man, my girl could throw a punch!
"Rob is doing fine now and my black eye will be gone soon... I hope."