Sources close to Kylie tell us she and her camp have seen the alleged sex tape that surfaced on a website Thursday, and their only reaction is ... it's 100% not Kylie doing the deed.

We're told she's scoffing, particularly because the blurry woman with blonde braids doesn't even look like her. Kylie was rocking that 'do and similar clothing a couple months ago on Snapchat. Whoever "leaked" the tape attached her Snap videos to make it seem like the whole thing was real.

But we're told the hair is the only thing the impostor nailed. Real Kylie didn't nail anyone ... on camera.