Aaron Michael , who finished 6th overall in "Project Runway" Season 19, is in police custody ... because he just got busted in West Virginia.

The designer was arrested Thursday on an out-of-state warrant ... according to court documents obtained by TMZ. Bravo

Aaron is being held at the North Central Regional Jail in Greenwood, where he posed for a mug shot in an orange jail jumpsuit.

It's unclear what the arrest warrant was for ... we're working on it.

Aaron was eliminated from "Project Runway" Season 19 in the penultimate episode, following a challenge where designers had to create an outfit inspired by a hairstyle.

Story developing ...

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