This is creepy, even for Zak Bagans ... he got his hands on John Wayne Gacy's brain -- a brain he wants to buy -- and says he could sense the evil that consumed the serial killer.

The "Ghost Adventures" star tells TMZ ... he held Gacy's brain in his hands during a recent trip to the Chicago home where the late serial killer's forensic psychiatrist, Dr. Helen Morrison, keeps the organ ... which is preserved in a container in her basement.

Gacy -- infamously known as the "Killer Clown" -- was executed way back in 1994 after being convicted of murdering 33 young men and boys during the '70s, burying most of the bodies in a crawl space under his Chicago-area home.

Zak says he will never ever forget what he felt as he cradled Gacy's brain the palm of his hand ... pure evil!!! Bagans adds he was shaking ... yeah, spooky.

Serial killers are kinda Zak's thing -- he's got tons of Charles Manson items in his Haunted Museum. He made a play to Dr. Morrison to sell him the brain, but she turned him down flat.

Dr. Morrison has owned the brain ever since it was sliced up during Gacy's autopsy and claims she feels his spirit following her ... and that's exactly why Zak wants to buy it and do further research at his haunted museum.