MLB Star Barry Zito -- Rent My Giant S.F. Mansion ... $25k Per Month!

He once signed a $126 MILLION contract ... but former MLB pitcher Barry Zito could still use a few extra bucks -- which is why he's trying to rent out his S.F. mansion for $25k per month.

The ex-San Francisco Giants hurler has listed his 2-acre estate in Marin County, CA to rent or buy ... the latter will run you about $9 million.

The place is pretty sweet -- 4 bedrooms, 5 bathrooms, a wine cellar, pool, outside fire-place with seating area, a guest house ... and privacy, privacy, PRIVACY!!!

The former Cy Young winner bought the place in 2007 for $8.8 mil ... but clearly, he's over it.

Zito's also selling another mansion he owns in the Hollywood Hills.