Officials at the Wisconsin state crime lab put the gun through a shredding machine last month, according to video obtained by NBC affiliate WTMJ-TV.
Mr Rittenhouse’s attorneys and state prosecutors agreed in January to destroy the gun. In court filings, the teen said he didn’t want someone to buy the weapon as a trophy.
“We don’t want anything to do with that,” Mr Rittenhouse said on The Charlie Kirk Show last year.
Previously, guns used in high-profile killings like the slaying of Black teenager Trayvon Martin have been auctioned off for hundreds of thousands dollars.
Kyle Rittenhouse used the weapon to fatally shoot two people, Joseph Rosenbaum and Anthony Huber, and wound a third person at February 2020 protests following the police shooting of Jacob Blake, a Black man.
In November, a jury acquitted Mr Rittenhouse on all charges related to the shooting, which he claimed was in self-defence.
"He was chasing me. I was alone. He threatened to kill me earlier that night. I didn’t want to have to shoot him," Mr Rittenhouse testified about one the shootings. "I pointed it at him because he kept running at me, and I didn’t want him to chase me."
Mr Rittenhouse, once part of a “public safety cadet programme” in the Chicago suburbs, idolised police and supported Donald Trump and the Blue Lives Matter movement, before heading to Kenosha, his social media shows.
Once there, he joined with other self-styled militia members to patrol outside of local businesses, at one point earning passing thanks from police officers who tossed him a water bottle and said, “We appreciate you guys. We really do.”
He was also recorded telling people he was a field medic, despite having no medical training.
During his trial, prosecutors showed a video from before the shootings where Mr Rittenhouse can be heard fantasising about shooting looters.
As he watches a group of hooded individuals loot a CVS store, he tells another person, "Bro, I wish I had my f***ing AR. I’d start shooting rounds at them."
At the time of the shooting, Mr Rittenhouse was 17, below the legal age to buy or possess an assault rifle in Wisconsin. A friend purchased him the weapon instead at a hardware store.