Smith's partner of five years, Kisha Chavis -- a former adult actress who went by the name Yasmine Pendavis -- shared footage of his reaction to discovering her XXX subscription-based profile ... and he was PISSED.

"Just finding out you got an OnlyFans page all these years," Smith said in the clip ... adding she should have had a conversation with him before hopping on the platform.

"That's bulls***," Smith -- the No. 1 overall pick in the 1995 NBA Draft -- said. "That's f***ed up, Kish. I'm telling you, that's f***ed up."

Kisha then defends herself ... claiming she isn't doing any work with other people, so it's "my body, my f***ing choice."

She also said she tried numerous times to come up with a solution to her financial status together ... but ultimately decided to revisit her former career path.

"You knew who the f*** I was when you met me ... I thought I would never have to go back to anything like this again. But, unfortunately, that's not the case right now."

It's worth noting -- Smith reportedly made $61 million in his career ... but only pocketed around $18 mil after taxes and agent fees, among other expenses. He has since opened up about his money problems.

Smith said he doesn't care how much money she's making on OF -- it doesn't change the fact that she was doing it behind his back ... and he sees it as a smack in the face.