Eleonora Antillón had kept her secret bottled up for more than 30 years.

But when she read an article on social media on Tuesday saying a woman had accused Óscar Arias Sánchez, the former president of Costa Rica, of sexual assault, she did not hesitate: “From personal experience with him,” she wrote on the social media platform, “I BELIEVE her!!”

Ms. Antillón is among four women in a 48-hour period to accuse Mr. Arias, 78, of unprovoked and unwanted touching or sexual assault.

The allegations are a serious blow to the legacy of Mr. Arias, who won the Nobel Prize in 1987 for his effort to bring peace to Central America. Twice president of Costa Rica, he has been one of Latin America’s most respected figures.

Ms. Antillón, a former talk show host known by her nickname, Nono, said in an interview that Mr. Arias sexually assaulted her in 1986, when he was a presidential candidate and she was a young press aide.

“I asked him some question, and instead of answering, he literally ran his hands over my chest and between my breasts and said, ‘You’re not wearing a bra!’” Ms. Daly said. “I was so stunned that all that I could think of was to say, ‘Yes, I am!’”

Although she told her boyfriend and other people when it happened, it never occurred to her to file a complaint against a sitting president, she said.

“I didn’t get physically hurt the way other people do,” she said. “It was the humiliation of being made to feel you don’t matter in any way.”

Marta Araya Marroni, a 53-year-old book editor, said she met several times with Mr. Arias as the editor of his 2012 collection of essays and speeches, Con Velas, Timón y Brújula. In one meeting, he suddenly put his hand on her leg. She brushed him off, telling him that he had a girlfriend and the move was inappropriate. She said he then called her twice, insisting that she come over to his house for a massage. Ms. Araya said she finally hung up on him.

“I never kept it a secret. I told everyone,” Ms. Araya said. “The only reason I would have to publish this is so that the people believe the women who file complaints. It disgusts me a lot that people always think these are lies.”

For Ms. Antillón, although the statute of limitations for sexual assault — 10 years in Costa Rica — is long past, she said she would be happy to make a declaration in court.

“I owe it to myself, and I owe it to a lot of women,” Ms. Antillón said.