'Burn Notice' Star Seth Peterson -- Check Out the 22-Year-Old I'm Banging ... NOT My Pregnant Wife

"Burn Notice" star Seth Peterson proudly plastered his new 22-year-old girlfriend all over the Internet -- which normally would be fine ... except he's still got a wife ... who's about to give birth.

TMZ broke the story ... Peterson's wife Kylee Cochran just filed for divorce, claiming her 43-year-old husband bailed on their 12-year marriage in September (when she was just 3 months pregnant with their 3rd kid) to shack up with Miranda Doerfler, a 22-year-old aspiring writer.

Miranda has since posted photos of herself with Peterson all over Facebook, writing captions like, "Okay. Officially. Seth Peterson and I are a couple."

One of Miranda's friends commented, "I thought he was married." And Miranda responded, "Lol, yeah" ... meaning, he WAS. Rhymes with witch.