A sex tape Blac Chyna made -- with an unknown guy -- has leaked online, but it doesn't appear she has anything to do with it, and she wants cops to track down whoever posted it.

The video shows Chyna performing MouthAction on the mystery man. His face is never shown, but you can hear his voice. Chyna's face is very clear when it's not being hidden by ... y'know ... Instruments. The 1 minute, 23 seconds long clip was first posted early Monday morning by an anonymous Twitter account, and appears to have been shot by the guy.

BC's attorney, Walter Mosley, tells us he can't comment on the video -- but added, "It's a criminal matter," and they're contacting police to investigate.

Worth noting ... Chyna claimed she was a victim of revenge porn last year when Rob Kardashian posted nude pics of her. Despite the fact she "liked" those images on Instagram, she had threatened to sue him.

We should say ... Rob's clearly NOT the guy in the sex tape.

Watch here http://skillzah.com/2018/02/blac-chyna-s3x-tape-leaks-video-here.html