The report of the twins' arrival brings Musk's total count of children to nine.
Billionaire tech entrepreneur Elon Musk had twins last year with an executive at one of his companies, online outlet Business Insider reported, citing Texas court documents.
The babies' mother, 36-year-old Canadian Shivon Zilis, is an executive at Neuralink, Musk's brain-implant maker, and has worked at multiple of his other companies, including OpenAI and electric car manufacturer Tesla, Business Insider said on Wednesday.
She and Musk, who became the richest person in the world last year, in April filed a petition with a Texas court for the children to "have their father's last name and contain their mother's last name as part of their middle name," Insider reported, referring to court documents obtained by the publication.
The petition was granted in May, the site said.
The babies, whom Insider reports were born in November, arrived just weeks before Musk, 51, and music artist Grimes had their second child via surrogate.
They welcomed a baby girl named Exa Dark Sideræl Musk – although the parents will mostly call her Y.
In total, Musk has fathered 10 children, one of whom died shortly after birth.
Slew of lawsuits
Early this year, the chief of Tesla and SpaceX, Musk launched a now-stalled $44 billion bid to buy Twitter, with Insider reporting that Zilis's name has come up as an option for who might eventually run the social media giant.
Tesla, which has relocated to Texas from California, has faced a series of lawsuits alleging discrimination and harassment against Black workers as well as sexual harassment.
And in May, the South African-born Musk himself denied allegations that he groped and exposed himself to a flight attendant six years ago, with news outlet Insider reporting SpaceX paid a woman $250,000 in 2018 to settle a misconduct claim against him.