Farrah Abraham's Sex Tape Bikini -- Can Be Yours ... FOR FREE!!!

Farrah Abraham's infamous bikini is no longer for sale -- that's because a porn site snatched it up and now plans to GIVE IT AWAY to one lucky(?) fan ... TMZ has learned.

As TMZ first reported, the bikini -- which Farrah wore on the cover of her sex tape -- was pulled off eBay because of "health and hygiene concerns" ... since the bikini had not been washed since Farrah wore it.

We're told Pornhub decided to buy the bikini from Vivid before it was re-listed, but it's unclear how much they paid for it (bidding had reached an insane $14,700 before it was pulled). The contest should be live today over at pornhub.com/win/farrah.

And in case you were wondering ... we're told the bikini has still not been washed.