Tupac Shakur's 1991 sex tape is officially off the market -- TMZ has learned, the XXX footage has been sold to a private collector.

Sources connected to the deal tells us, the former tape owner decided not to sell to a big company out of principle -- claiming the corporate porn fat cats were "parasites."

No word on what it sold for ... but as we previously reported, bids for the background music alone -- an original, unreleased Tupac track -- had reached $1,000,000.

Maybe next time, folks ... though we wouldn't hold our breath -- a rep for the Tupac estate tells us, it plans to block any future sale of the tape ... and make sure the tape never sees the light of day.