Justin Bieber could have a brand new porn-star neighbor in his ultra-exclusive L.A. community -- 'cause TMZ has learned Farrah Abraham has her eye on a $3 million pad right around the corner from the singer.

We've learned ... the star of "Back Door Teen Mom" was on the mansion hunt this weekend in The Oaks in Calabasas -- an expensive gated community home to stars like Kourtney Kardashian, Travis Barker, Romeo, Keyshawn Johnson, Shanna Moakler and others.

One of the homes that tickled Farrah's fancy was a 4,851 square feet pad with 5 bedrooms, 6 baths ... and a $3,299,900 price tag.

It's unclear if Farrah has already put down an offer ... but with the 6-figures she made from the tape, she could definitely afford the down payment.

The monthly mortgage payments, on the other hand ... let's just say she might need to re-open the back door if she plans on staying here a while.