Johnny Galecki remembers how Kaley Cuoco first hit on him

Kaley Cuoco used a “Big Bug Theory” to lure her co-star and crush Johnny Galecki to her cabin, a new tell-all reveals.

Galecki — tight-lipped until now about his off-screen romance with fellow “Big Bang Theory” cast member Cuoco — dished that she first hit on him by using a ruse about an insect.

They were on a cast getaway at the San Ysidro Ranch in Montecito, Calif., and Cuoco, who had just broken up with her boyfriend, was staying in the room next door.

“She tried to have him come over in the middle of the night to kill a bug,” said Jessica Radloff, author of “The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series,” out Oct. 11.

“So I declined the demand to go kill a bug in her room,” Galecki recalled in the book. “But apart from the bug, I smelled a rat and was thinking, ‘Is this your way of inviting me into your hotel room basically?’”

Cuoco admitted to the author her motives were twofold.

“There was definitely a bug in my room — and I was flirting with him!” she said.

The former flames met on the set of the beloved sitcom, where they played the seemingly mismatched couple of Leonard, an experimental physicist, and Penny, a waitress and aspiring actress.

They acted together for the show’s entire 2007-2019 run. By the sixth episode of the first season, they were already falling in love for real.

At first they tried to hide it from the cast.

“They never had sex on the set or in their dressing room,” Radloff told The Post. “They were trying to sneak around, but people were very aware that something was going on, so they weren’t as good as they thought they were.”

Galecki, now 47, “cried a couple times” when he spoke about Cuoco, 36, for the book, Radloff said.

The pair agreed on mostly everything — but had different versions of what happened after their first kiss.

“After that kiss … she said, ‘You gotta come over to my house,’ so I did,” Galecki said. “And then she disappeared. She came back having showered, wearing nothing but a towel, and sat on my lap.”

Cuoco claims she was fully clothed underneath the towel.

The discrepancy still puzzles Galecki and the author.

“Kaley thinks, ‘Of course I had clothes under the towel.’ But as Johnny says — which is true — ‘Who wears clothes under a towel?’”