'Teen Mom' Star Kieffer Delp Arrested for Operating Meth Lab

"Teen Mom" star Kieffer Delp may be back to his old ways of running afoul with the law ... cops arrested him for allegedly operating a meth lab.

Kieffer -- one of Jenelle Evans’ earliest boyfriends on the show -- was arrested after authorities in nearby Pittsburgh uncovered a suspected meth lab. Cops tell us they were called to an apartment building late Wednesday night and made the discovery.

Cops evacuated the building during the arrest because of a strong, lingering odor inside.

As we've reported ... Kieffer is a known drug user and he's previously been arrested for punching a girl and her male friend.