Mischa Barton Says She's the Victim of Revenge Porn

Mischa Barton says someone she dated has betrayed her in the most personal of ways ... shopping around what she says is revenge porn.

Mischa and her lawyer, Lisa Bloom, just warned the suspect ... the law will come down on him hard.

The star of "The O.C." says the ex used a hidden camera and videotaped her without her consent. Bloom says the guy has been shopping the tape around to porn sites. She says she's obtained a restraining order against the guy, who apparently is looking for a big payday.

Bloom has contacted cops who are investigating.