Matt Kemp may be on the disabled list ... but his wallet sure isn't.  The L.A. Dodger is forking over $30,000 A MONTH to live in Tamar Braxton's old mansion, TMZ has learned.

We broke the story, Tamar and her hubby -- bigwig record producer Vincent Herbert -- sold their Hidden Hills, CA home for $6.9 million earlier this year ... scoring a $3.5 million profit.

Now we've learned Matt signed a one-year lease for the 11,700-square-foot digs from the new owner -- real estate mogul Carlton Gebbia from "Real Housewives of Beverly Hills."

As for Matt's temporary crib ... it has 5 bedrooms, 7.5 baths, home theater, espresso center, a mirrored elevator, massive pool and spa.

FYI -- Matt already owns a $2 million casa in Arizona.