The 64-year-old joined the cast of the West End musical to perform his classic hit "School's Out"—marking the anniversary of the track hitting No. 1 in the U.K. charts 40 years ago this week.
The audience members appeared delighted, shouting, dancing and singing along with him. Those who hope to catch him again will be disappointed, though. He said the appearance was a one-off.
As far as musicals go, "Rock of Ages" seemed a good fit for Cooper. The show, which was recently adapted for the big screen with Tom Cruise, is about an aspiring rock star and features classic rocks hits from bands including Bon Jovi, Journey and Poison.
"The songs that they're playing—everybody knows every word of every song," Cooper noted in an interview with the Associated Press ahead of the show.
He also talked about how it felt to crash a musical.
"It's different because it's not my show so you do feel like a bit of an intruder, but at the same time it complements the show so much," he said.
Cooper also reflected on the enduring popularity of "School's Out," which first became a success in London despite—or because of—calls to ban his group's concerts due to their shocking theatrics inspired by horror movies.
"I knew it was going to be a hit," Cooper said. But, he added: "I did not know it was going to have the longevity, it was going to be the national anthem.
The London production of "Rock of Ages" has been running at the Shaftesbury Theatre since September 2011.