The stars, who were married for four years before their divorce in 1994, have traded insults online and in the press since their split, and last year (11), Arnold admitted he hadn't spoken to his former partner in person for 18 years.
But that all changed on Saturday as Arnold joined Seth Green, Katey Sagal, Ellen Barkin and host Jane Lynch to poke fun at Barr for the Comedy Central TV special.
Glee star Lynch opened the show by quipping, "She's one of those rare celebrities so famous that she's referred to by just one name - b**ch. I waved to you outside but then I realised it was just one of those inflatable parking lot gorillas."
Funnyman Jeffrey Ross then tore into Barr over her 2012 bid for the White House, saying, "Instead of running for president, why don't you try walking on a treadmill. I'd vote for you but I'm against big government. That's just what we need, a round president in an oval office."
The laughs continued until towards the end of the show, when a stunned silence fell over the crowd as Arnold appeared onstage to add his comedic insults to injury.
Kicking off his set, he said, "I'm not here to tear Roseanne a new one, because I've seen the old one", but Arnold soon softened and proved he was ready to put their bitter relationship firmly in the past and start afresh by hailing the comedienne for her groundbreaking career.
He told the audience, "In 1985, Roseanne went on Johnny Carson, which is every comic's dream back then. She killed, he laughed his ass off, he gave her a thumbs up and he invited her to sit on his couch. She got validation from the king which can never be taken away from her.
"And I just want to say, Rosanne, you were my Johnny Carson. And thank you for the thumbs up and thank you for inviting me to sit on your couch for a little bit."
The Comedy Central Roast of Roseanne will air in the U.S. on 12 August (12).