Tom Arnold says his ex-wife, Roseanne, is undeniably racist ... also acknowledging she's plagued by mental illness.
Tom was on "Good Morning Britain" Monday when Piers Morgan asked him about the tweet that tanked the show. He says Roseanne has been off the rails the last 6 months ... so he's not surprised.
He says Roseanne suffers from mental illness -- multiple personality disorder -- but that doesn't excuse what she said. He talked about her meds, but didn't think the Ambien defense holds water.
On the subject of Trump ... Tom, who appeared on "Celebrity Apprentice," says no question there ... the Prez is a racist who is fueling racism. He has claimed during the filming of the show, Trump said racist things in outtakes.