The actress/singer has revealed she collects the deadly weapons and takes her favourite everywhere with her - even carrying the knife on a flight across America.
She tells Britain's Metro newspaper, "I have a knife collection. I have my favourite black knife with me all the time. It's a switchblade. It relaxes me to flick it. I close it and open it.
"I flew from New York to LA and still had a couple of knives in my purse. I thought I took them out, but I opened my bag in LA and they fell out. I was like 'holy s**t.'"
But the comments have landed the 16 year old in trouble with Britain's Mothers Against Murder and Aggression - a charity which helps support families who have lost loved ones through violent crimes.
Spokesperson Lyn Costello has called Momsen's attitude "warped”, insisting she needs to "get a brain". She adds, "We’re losing our teenagers on a high scale to knife crime.”