Justin Bieber -- Paying Mom's Rent ... And Mom's In Trouble!

Justin Bieber doesn't get drugs from his mom for free -- TMZ has learned, the singer has been paying rent for his mother's secret L.A. mansion since 2012 ... and the place is sweeeeeeeet.

No one ever knew JB's mom Pattie Mallette had a place in L.A. ... let alone one so badass.

Turns out ... the tree doesn't fall far from the apple, because Pattie got herself into some trouble over the 2,900 square foot house.

Seems Pattie wanted to do some home improvement in her home gym but never got the permission of her landlord. She had a mirror installed last summer, but didn't pay the bill. So the company that did the work has filed a lien against the property ... and her landlord was none too pleased.

The lien was for a modest $3,414. We're told Pattie thought she was being overcharged and refused to pay. Pattie settled up a week ago for around $2,600. And TMZ informed the landlord Friday and he was PISSED!

As for who's paying the $8,000 monthly rent ... we've learned one of Justin's companies is handling it.