Christie Brinkley described the moment she discovered her fourth husband, Peter Cook, was having an affair with a teenager.

In her newly published memoir “Uptown Girl,” the model disclosed that she first became aware of Cook’s unfaithfulness when a stranger approached her following a speech she had given at a high school graduation in the Hamptons in 2006.

After giving the graduation speech, the stranger approached her and in a soft tone said, “Excuse me,” as detailed in her book. He continued, “I need to tell you that your arrogant husband has been having an affair with my teenage daughter.”

Brinkley, aged 71, recounted that she was left in a state of shock and disbelief, feeling overwhelmed by the revelation. She recalled how she froze momentarily, completely taken aback, and then turned to look at Cook, who was present in the audience alongside her son, Jack, now 29, whom Cook had legally adopted.

“As soon as I saw him, Peter’s eyes were already trained on my face, as he began shaking his head, ‘No,’” she recalled.

“When your whole world falls apart and you realize in a heartbeat you’ve suddenly become the cliche middle-aged woman whose husband is having an affair with a much (much) younger woman, what do you do?” the actress wrote.

That July, Brinkley announced that she and the architect, now 66, who also share daughter Sailor, 26, were divorcing after 10 years of marriage.

The exes then went through a contentious court battle over their two children.

“I kept saying I don’t understand what good was a prenup?” the former Sports Illustrated Swimsuit Issue cover model wrote.

“It was exhausting and scary because I always lived in fear of them taking my kids because that was the constant threat and a terrifying thing to live with. I got a prenup so I didn’t have to go through all this but still did. It was mind-boggling and it was hard to write about.” 

Their split was finalized four years later, with a judge urging the couple to settle their custody battle for the sake of their kids.

Brinkley — who also shares daughter Alexa Ray, 39, with her second husband, Billy Joel — wrote in her book that there was a time when she wondered if she “would ever recover.”

“Whenever I felt this, I stopped, looked inward and counted my blessings, which was a long list that always started with Alexa Ray, Jack, and Sailor,” she shared.

Brinkley’s memoir, “Uptown Girl,” is now available for purchase.