Alex Rodriguez -- $15 MILLION PROFIT in Miami Beach Mansion Sale

He might not be great in October, but Alex Rodriguez is CRUSHING IT in May ... 'cause the baseball star just sold his Miami Beach mansion for a $15 MILLION profit, TMZ has learned.

We know ... A-Rod has accepted a $30 million offer for his 19,861-square-foot Miami Beach estate ... the same property he purchased in 2010 for $7.4 mil.

After he bought the place, the NY Yankees star sunk an additional $7.6 million into building his dream home -- meaning his total investment was $15 mil.

The home is unreal -- an oceanfront palace with 9 bedrooms, 11 full bathrooms (plus 2 half bathrooms), a 4-car garage, state of the art security system with 16 cameras and a Zen garden.

Plus, it's on the same street as Matt Damon!

Rodriguez quietly put the place on the market in August 2012 and was asking for $38 mil -- but ultimately, he decided to settle for a measly $30,000,000. Bummer ...

So who bought the joint? Unclear at this point ... but sources tell us the buyer is a celebrity who lives in Palm Beach.