Conrad Hilton Arrested -- Behind Bars After Weed Arrest

Conrad Hilton -- Paris Hilton's youngest brother -- is currently behind bars in juvenile hall in Downtown L..A. ... law enforcement sources tell TMZ.

We're told 18-year-old Hilton was arrested after testing positive for marijuana ... a violation of his probation stemming from a previous marijuana arrest.

Hilton will remain in custody until he gets before a judge ... and considering it's Friday evening, he might be stuck behind bars until Monday.