Blac Chyna's most recent sex tape partner is her ex-boyfriend, Mechie ... according to him, and even though he admits recording it himself, he's as pissed as she is about the leak.
A rep for Mechie tells TMZ ... the explicit video showing a naked Chyna performing oral sex was shot back in July. We're told Mechie was the one who recorded the encounter, but it was all captured using Chyna's phone.
His rep says Mechie never had his own copy of the video, so he has no clue how it got out.
You'll remember -- Rob Kardashian posted naked pics of Chyna last year ... as well as an intimate video of Mechie making out with dark-haired Chyna. The sex tape is from a different session because Chyna has a platinum blonde 'do.
We broke the story, Chyna's attorney, Walter Mosley, calls the new, leaked video a "criminal matter" -- while her other lawyer, Lisa Bloom, put a name on the crime ... revenge porn.
Our sources say Mechie plans to go to cops, and wants the leaker found and prosecuted.
We're told he and Chyna have not talked since their split in August.