Lisa Birnbach planned to take her friend’s secret to the grave.

When E. Jean Carroll called more than two decades ago to tell her that she’d just been sexually assaulted by Donald Trump in a Manhattan department store dressing room, Birnbach said she begged Carroll to go to the police. When her friend resisted, Birnbach swore she’d never tell anyone.

Even after more than a dozen women came forward in 2016 to accuse the then-presidential candidate of kissing and groping them without consent, Birnbach said she didn’t think about the episode again until Carroll told her last year that she had decided to write about it in a new book.

“I know a lot of women he has hassled,” Birnbach said.

Trump has vehemently denied the claim by Carroll, a longtime Elle magazine columnist who went public last week with her description of the alleged assault. She is one of 16 women who have accused him of sexual misconduct, allegations he says are untrue.

Birnbach, a New York-based author, was one of two friends who told The Washington Post that Carroll had described the assault to them shortly after it allegedly happened. They both spoke on the condition of anonymity at the time to protect their privacy.

After Trump and his allies attacked Carroll and her account, Birnbach and the second friend, former New York television anchor Carol Martin, decided to come forward this week, first speaking on the record to the New York Times.

“Apparently my validation makes E. Jean more believable, and my only goal is to be an honest person who tells the truth,” Birnbach said. “I told the truth not as a political observer — I told what happened to my friend 23 years ago.”

Martin said she was working with Carroll at a cable news station at the time of the alleged incident, which she said Carroll described to her as they sat in Martin’s kitchen days later.

“She was dumbstruck,” Martin said.

The decision by her two friends to go public comes as Carroll’s allegation has stirred little political response, even among the president’s critics. House Speaker Nancy Pelosi (D-Calif.) told reporters Thursday that she had not followed the story closely.

Birnbach, a former correspondent for CBS’s “The Early Show” and co-author of “The Official Preppy Handbook,” said she wasn’t bothered that the Democrats were not focusing on the episode amid the turmoil of the Trump era.

“There will be another scandal breaking by dinner time, so you think, ‘Don’t go all in on this story, save some energy for the next story,’ ” she said, adding: “There are so many crises. You have to do triage on your emotions.”

Carroll has said that during a chance encounter at a Bergdorf Goodman department store in New York in late 1995 or early 1996, Trump attacked her in a dressing room. Carroll said he knocked her head against a wall, pulled down her tights and briefly penetrated her before she pushed him off and ran out.

Birnbach, who at the time was working as a TV journalist, said Carroll called her moments later, laughing almost manically. Birnbach told her friend she was describing rape and urged her to go to the police.

In an interview last week, Carroll attributed her reaction to adrenaline.

“You either laugh or you cry, and if you cry you’re going to double the burden, so I laughed,” she said. “And women for centuries, this is how we deal with it. We don’t want to upset the man — he’s bigger and more powerful.”

Trump has said Carroll is “totally lying” and added that she was “not my type.” His allies, such as Fox News host Tucker Carlson, have sought to discredit Carroll’s story, questioning why she did not come forward sooner.

Carroll told The Post that, like many women who have been assaulted, she blamed herself.

“It’s such a personal decision, when you come forward and when you don’t come forward,” she said. “It’s so personal.”

Beth Reinhard contributed to this report.