Years before Vanessa Trump filed for divorce from Donald Trump Jr., their marriage was rocked when — around the time she was pregnant with their third child — he cheated on her with a contestant from “The Celebrity Apprentice.”

Page Six has exclusively learned that Don Jr. — then a so-called “adviser” on the NBC show — fell for busty Danity Kane star Aubrey O’Day while filming “Celebrity Apprentice” in 2011.

Sources say that Vanessa — who filed for divorcefrom President Donald Trump’s eldest son last week after 13 years of marriage — was devastated when he told her that he planned to leave her for O’Day.

Vanessa was pregnant with their third child, Tristan, around that time.

Don Jr. “pursued [Aubrey]. It was him who chased her,” said a source familiar with the situation. “He told her that his marriage was already in the process of dissolving.”

But the insider added, “I think his marriage to Vanessa was over long before Aubrey came along.” O’Day’s rep declined to comment.

A second source said that Trump Sr. stepped in when he heard about the relationship with O’Day and told Don Jr. to “knock it off.”

The first source said that his family “pressured [Don Jr.] to stay in his marriage.”

Sources say that the pair would meet up in various cities during their fling and wrote each other love notes.

On election night in 2016, O’Day — who became famous after appearing on Diddy’s reality show “Making the Band” in 2005 — pointedly tweeted, “my story I didn’t tell is worth millions now.”

However, sources say that O’Day was truly in love with Don Jr., that she never sought money either to tell her story or to keep quiet, and that she was never paid by the Trump family. Suggesting an unusual taste in men, she later went on to date “Jersey Shore” star Pauly D.

A third source said that Vanessa “was going to leave [Don Jr.] then. That was the start of the downfall [of their marriage].”

Don Jr. and Vanessa have five children, and she filed for divorce last Thursday.

Reps for the ex-couple didn’t respond to our repeated requests for comment.