Zak Bagans just bagged some historical artifacts for his famous museum ... a couple of Saddam Hussein's belongings, courtesy of "The Sheriff of Baghdad."

The "Ghost Adventures" star tells TMZ ... retired U.S. Army Special Operations Sergeant Major John "Shrek" McPhee brought him Saddam's hat and sweater all the way from Iraq.

John obtained Saddam's clothing directly from Hussein's tailor in Baghdad, before U.S. forces captured Saddam in 2003.

John's arguably the most badass U.S. military member in the last 30 years, and Zak says they became friends recently after Joe Rogan put him in touch with "The Sheriff."

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Zak says he's going to collab with John on a Civil War battlefield investigation for his show ... and John gave him the Saddam gear as part of their deal to do the investigation together.

John's a big "Ghost Adventures" fan himself ... he told Zak he would binge-watch Zak's show when he came home from his deployments.

No word on a release date for Zak and John's collaboration ... but in the meantime, Zak says he's going to put the Saddam clothing on display in his Haunted Museum in Las Vegas.