"I've been dieting since we heard that Cher was coming, so I can fit into the outfit," Underwood said.
The comedian went with the singer's getup from the "If I Could Turn Back Time" music video, while Chen opted for the unforgettable Bob Mackie outfit from the Oscars, Gilbert went with the Sonny and Cher era, Tyler took on '70s Cher and Osbourne dressed like Cher from her farewell tour.
"It's every girl's dream to be Cher for a day," Chen admitted.
Cher was with the ladies for the full hour of the show, and she revealed details behind her 1989 "If I Could Turn Back Time" ensemble that were controversial at the time.
"It's almost nothing now," Cher pointed out. "It looked really daring, but if you were wearing a bathing suit, you're kind of seeing the same thing. It was just the way Bob cut it. It just made it look like, 'Oh this is just too much.'"
"I had such a great time doing that, and I was working with all the sailors and they were calling me ma'am," she continued. "We had a whole scenario. I was on a boat and I was in a carriage. It was a whole thing and then they cut all of it out and said, 'That's what we want to see! Get on that cannon, baby!'"
The GRAMMY winner just announced 18 added shows to her Classic Cher show at the Park Theater at the Monte Carlo Resort and Casino in Las Vegas. She also has March shows scheduled at MGM National Harbor in the Washington, D.C. area.
Tune in to ET tomorrow for our exclusive interview with Cher!