'Criminal Minds' Star Thomas Gibson -- I Didn't Cheat on My Wife ... We're Separated

"Criminal Minds" star Thomas Gibson wasn't pulling a fast one on his wife by conducting an online relationship with a shameless catfisher -- sources tell TMZ, the actor and his wife have been separated for years.

Sources close to the couple tell us, Gibson and his estranged wife Christine Gibson still live together for the sake of their children, but they've been separated for 2-and-a-half years ... six months before Thomas started messing around with one of his Twitter fans.

As we reported, Gibson and the woman -- who catfished him by posing as a porn star -- carried on a heated relationship for two years before Gibson discovered the entire thing was a sham ... he even sent her a sexy hot tub video (it's hilarious).

Bottom line: Gibson might be a sucker ... but he's no cheater.