EXCLUSIVE

Justin Theroux is the most dedicated celeb fan of Howard Stern -- that's the only reasonable explanation for the giant prosthetic face now hanging on his wall.

Theroux scored a lifecast of Stern Wack Packer, Beetlejuice ... from special effects artist Mike Marino of Prosthetic Renaissance. Marino says Justin was in his studio a couple weeks ago when he saw the mold and freaked out.

We knew JT was tight with Stern -- who spoke at his wedding to Jennifer Aniston -- but we didn't know it was this deep. Marino says Justin immediately recognized the Beet mold from 2004, when he first made it on the 'Stern Show.'

He says he brought it out of retirement, made a replica, painted it bronze and shipped it to Justin as a gift. Jennifer's gonna love it.