Lil Durk has been placed in solitary confinement at the Metropolitan Detention Center in L.A. ... TMZ has learned.

Jose Rojas, a union secretary representing Bureau of Prisons employees, tells TMZ ... Durk was placed in the SHU at MDC LA on Friday.

Here's the deal ... sources with direct knowledge tell us ... Durk's cell has been "constantly searched," but prison authorities haven't found any contraband since he was acquitted in the murder-for-hire case.

We're told Durk was sent to the SHU because he'd asked the deputy warden about all the searches ... and was allegedly reprimanded for "talking back."

Meanwhile, the Bureau of Prisons is staying mum on the situation.

A BOP spokesperson tells TMZ ... "For privacy, safety, and security reasons we do not discuss the conditions of confinement for any individual in our custody, including housing assignments, potential disciplinary infractions, or internal security measures."

As you know, Durk was found not guilty in his murder-for-hire trial back in September. He still faces felony charges for racketeering and firearms.

His second trial date was originally scheduled for this past month ... but got pushed to next August.