The child is called Tau Techno Mechanicus, the technology billionaire wrote in a post on X (formerly Twitter) on Sunday.
Musk, who owns the social network, was responding to a post from entertainment news account Pop Base, which named his and Grimes’ three children as “X Æ A-Xii, Exa Dark Sideræl Musk, Techno Mechanicus.”
The child’s existence had been kept a secret until recently.
Musk and Grimes broke up in September 2021, but the Canadian singer – born Claire Boucher – has described their relationship status as “very fluid.”
In a March 2022 interview with Vanity Fair, she revealed that Exa Dark Sideræl Musk was born via surrogate in December 2021.
Although the couple apparently had no intention of announcing the birth of their second child, nicknamed “Y,” Grimes shared the news after Vanity Fair journalist Devin Gordon overheard a baby crying.
Grimes and Musk previously welcomed a son named X Æ A-Xii in May 2020.
Musk, 48, has five other children – twins Griffin and Vivian and triplets Damian, Saxon and Kai – from a previous marriage.
He has previously spoken out about his concerns over declining birth rates.
“USA birth rate has been below min sustainable levels for ~50 years,” Musk wrote on Twitter in May 2022.
And he reiterated his stance in July that year.
“Doing my best to help the underpopulation crisis,” he posted. “A collapsing birth rate is the biggest danger civilization faces by far.”