Elon Musk’s friendship with Sergey Brin has been strained by the alleged affair.

Mr. Musk is the world’s richest man with an estimated fortune of $240 billion, while Mr. Brin ranks eighth globally at $95 billion, according to the Bloomberg Billionaires Index.

Among the country’s most prominent entrepreneurs, Messrs. Brin and Musk have been longtime friends. Mr. Musk said that he had regularly bumped into Mr. Brin’s home in Silicon Valley over the years.

Mr. Brin gave Mr. Musk about $500,000 for Tesla during the 2008 financial crisis as the company struggled to ramp up production. In the year In 2015, Mr. Musk gave Mr. Brin one of Tesla’s first all-electric sport-utilities.

Tensions have been rising between the two men and their factions in recent months, people familiar with the matter said. Mr. Brin instructed his financial advisers to sell his personal investments in Mr. Mook’s companies, some of those people. It is not known how big those investments were or if there were any sales.

According to Santa Clara County Superior Court records, Mr. Breen filed for divorce from Nicole Shanahan in January of this year, citing “irreconcilable differences.” The divorce filing came several weeks after Mr. Breen learned of the brief affair, those people said.

At the time of the alleged affair in early December, Mr. Breen and his wife had separated but were still living together, a person close to Ms. Shanahan said. In his divorce filing, Mr. Breen cited December 15, 2021 as the date the couple would separate.

A lawyer for Mr. Breen declined to comment. Mr. Musk did not respond to requests for comment. A spokesman for Ms. Shanahan, who runs a foundation focused on reproductive justice, did not respond to requests for comment.

In an interview with the Puke news website earlier this month, Ms Shanahan said of the divorce filing: “I hope for the sake of our son, Sergei and I, that we can move forward with dignity, integrity and harmony. We are both working towards this.

In the past couple of months, Mr. Musk’s personal life has attracted a lot of attention. He was accused of exposing himself to a flight attendant at his aerospace company, SpaceX. The publication Business Insider reported late last year that he fathered two children with a female executive at another company, Neuralink. And one of his 10 children publicly disowned him.

Earlier this month, Mr. Musk sought to back out of a deal to buy Twitter, saying the company did not provide the necessary data to assess the spread of fake or spammy accounts. Twitter said it “bent over backwards” to provide the information. He sued Mr. Mook to force him to abide by the agreement, and a Delaware court agreed to an expedited trial in October.

Mr. Brin and Ms. Shanahan, his wife of nearly four years, met at the yoga retreat Wanderlust seven years ago, according to a person familiar with their relationship. Both were previously married: Mr. Brin is the CEO of 23andMe Inc. To Anne Wojcicki, founder of the genetics company, and Ms. Shanahan, financial executive. Mr. Breen has two children with Ms. Wojcicki.

Mr. Brin and Ms. Shanahan were facing problems in their marriage in late 2021, mainly due to the shutdown caused by the Covid-19 pandemic and caring for their 3-year-old daughter, according to people familiar with the relationship.

The meeting with Mr. Musk took place at an Art Basel event in Miami in early December 2021, those people said. Art Basel is a multi-day, annual festival that attracts an affluent audience from around the world.

At a party earlier this year, Mr. Musk knelt before Mr. Brin to apologize for his wrongdoing and ask for forgiveness, people familiar with the matter said.

Mr. Brin acknowledged the apology, but still does not speak to Mr. Musk regularly, those people said.

The alleged affair comes after Mr Musk broke up with singer Grimes’ on-again-off-again girlfriend in September. Mr. Musk and Grimes, whose real name is Claire Boucher, gave birth to a daughter, a daughter, in December 2021 via surrogate. Mr. Musk was born to Shivonne Zillis, a Neuralink executive, also last fall.

Mr. Brin and Ms. Shanahan are now involved in divorce mediation, with Ms. Shanahan seeking more than $1 billion, according to people familiar with the negotiations.

The two sides have yet to reach an agreement, with Mr. Breen saying Ms. Shanahan wants more than what her prenuptial agreement allows, the people said. Ms. Shanahan’s side argues that her claim is only a fraction of Mr. Brin’s $95 billion fortune and that she signed the prenuptial agreement while pregnant.

Mr. Brin Google now founded the Alphabet unit Inc.,

GOOG -5.81%

In the year with Larry Page in 1998, and helped make it one of the world’s most valuable companies. He and Mr. Page

Since then, he has been heavily involved in physical activities, at one point he tried to learn many different Olympic sports, people who know him. He runs a $4.4 billion family foundation that supports causes such as education and Parkinson’s research, and is associated with an airship startup called LTA Research and Exploration. He is currently writing a physics textbook.

– Jim Oberman, Rebecca Elliott and Tim Higgins contributed to this article.

Write Kirsten Grind at kirsten.grind@wsj.com and Emily Glazer at emily.glazer@wsj.com