Kaczynski was held in a maximum security prison in Colorado, but in December 2021 he was transferred due to poor health to a medical facility in North Carolina, US media reported.
"On Saturday, June 10, 2023, at approximately 12:25 a.m., inmate Theodore John Kaczynski was found unresponsive at the Federal Medical Center (FMC) Butner in Butner, North Carolina," a statement released by authorities reads.
"Responding staff immediately initiated life-saving measures. Staff requested emergency medical services (EMS) and life-saving efforts continued. Mr. Kaczynski was transported by EMS to a local hospital and subsequently pronounced deceased by hospital personnel."
Between 1978 and 1995, Kaczynski launched a nationwide mail bombing campaign, sending explosive devices to universities and offices of air carriers, in a bid to attract attention to what he believed was the erosion of human freedom by modern technologies.
Sixteen bombs sent by Kaczynski killed three people and injured over 20.
Having evaded law enforcement for years, Kaczynski eventually came under police radar after his brother, David, provided officials with various clues, including letters and documents that investigators matched to his 35,000-word manifesto.
It wasn't until April 3, 1996, that authorities arrested Kaczynski, nearly 20 years after his first homemade bomb exploded at a university in Illinois. By 1998, he was sentenced to life in prison.