Real Estate Mogul Kurt Rappaport Shells Out $3.7 mil for Rare Honus Wagner Card

The mystery buyer who dropped $3.7 MILLION on a rare Honus Wagner card is revealing himself ... and it's real estate mogul Kurt Rappaport!

The card is the legendary 1909-11 T206, which Rappaport describes as "the HOLY GRAIL -- the rarest and most desired of all sports cards."

The purchase -- handled by Heritage Auctions -- makes Rap's Wagner card the 2nd most expensive baseball card ever purchased ... behind a rare Mike Trout card that sold for $3.9 million back in August.

There are less than 60 Wagner cards in existence -- but what makes Rap's card extra special is the fact it's in such great condition.

Professional Sports Authenticator (known in the industry as PSA) graded Rap's card as a 3 -- which is pretty good for a card that's more than 100 years old.

"Just to put it into perspective, 3 means very good condition," Rappaport told the guys on "TMZ Live."

"The 50 or 60 that exist in the world, this is one of the Top 5. There's only 4 other cards that have graded any higher than this. These don't come up to sale and I had the opportunity to buy it and I did."

Rappaport doesn't consider himself a hardcore card collector -- just a guy who appreciates rare cards ... comparing them to classic art pieces.