The comedian, 64, dated the actor, 55, in the 1990s, and she has now posted a TikTok video recalling his unconventional approach to bathroom etiquette after she once showered at his apartment in Vermont.
She said in her clip: "One night, I spent the night there, and the next day I'm in the shower. "I get off on the bathmat, and I said, 'I need a shower towel,' and he goes, 'You're standing on it.'"
Griffin also claimed Black had just one bath mat that doubled as a towel, adding to her fans: "You guys, that's straight guys."
Despite the hygiene concerns, Griffin admitted she continued seeing the School of Rock star, saying: "So naturally I kept sleeping with him."
Griffin explained that at the time, Black was the most impressive person she had dated.
"He was the first boyfriend I ever had where other bros thought I was cool because I was dating him," she said.
Griffin also stressed she was using "dating" in a "very loose" manner.
To prove their past romance, the My Life on the D-List star shared a throwback photo of herself and Black lounging together on a couch.
She captioned the TikTok post: "You're not gonna believe this one. This is why you're never gonna wanna miss an episode of my new YouTube show Kathy Griffin: Talk Your Head Off'"
Fans quickly reacted to the post, with one asking: "I'm actually jealous. I love Jack Black. Did y'all laugh all the time?"
Other comments from fans included: "The towel thing is so real. So is still sleeping with him lmfao," and "I fear we've all been Jack Blacked at least once, Kathy."
Griffin has previously been romantically linked to Quentin Tarantino, Fred Stoller, Conan O'Brien, Leif Garrett and Steve Wozniak.
She was married to Matt Moline from 2001 to 2006 and Randy Bick from 2020 until their separation in January.
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