Jessica Simpson says Tony Romo’s suspicions of her cheating on him with John Mayer is the reason they split up after two years in 2009.

Throughout her memoir, “Open Book,” she describes her tumultuous on-again off-again relationship with Mayer, in which he would come in and out of her life, always ending their relationship via email.

In the fall of 2007, she was alone again and started dating the former Cowboys quarterback.

“If John texted or emailed, I would hold up my phone and tell Tony immediately,” she writes. “He knew the hold John had over me, and it was like telling your sponsor when you’re triggered.”

Simpson, 39, explains that Mayer stayed close to her family by way of his friendship with her sister Ashlee Simpson and her ex-husband Pete Wentz. Simpson claims that her father brought Mayer over to their home in Los Angeles when she was in town.

“By a fire in the backyard, he stood and told all of us that he loved me and that we could all trust him to be a good man. I told him I would always love him, but I was with Tony,” she said. “But I didn’t tell Tony. I broke my own rule of full disclosure about any contact, even accidental, with John.”

Soon after, Romo — who’s now married to Candice Crawford — saw an email from Mayer on Simpson’s phone and suspected they were seeing each other behind his back.

“I hadn’t cheated on Tony at all, but I could not lie and say I hadn’t even seen him,” she said. “And within that second, he broke up with me right there. Two years, gone with an email.”

She added that Romo eventually realized she was being truthful, but it was too late. After Romo, she went back to Mayer for a few months, ending things for good following his February 2010 interview with Playboy, in which he said sleeping with her was “sexual napalm.”

“When he reached out to me, I changed my number and changed my email,” she wrote.

Earlier this week, she told “Today” co-anchor Hoda Kotb that she’d gone back to Mayer “close to nine times” over the course of their relationship.

In her book, she explained, “Sometimes it was out of the blue, other times I knew it was coming, because my light would start to go dim. John loved me when I was shining, and he drew strength and inspiration to write from that light.”

In the memoir, which came out on Tuesday, Simpson also said she turned to alcohol to cope with her anxiety while dating Mayer.

She told the New York Times she doesn’t care what Mayer thinks of her book: “I don’t think he’ll be shocked. He knows these stories.”

Reps for Romo and Mayer did not immediately return our requests for comment.

Listen to this episode of Page Six’s podcast “We Hear” for an interview with Jessica Simpson’s former publicist Rob Shuter: