The 52-year-old billionaire entrepreneur shared the news in his new biography - and went on to confirm his new baby's name on X.

The little boy is called Techno Mechanicus, or Tau for short.

The word tau means multiple things, including the 19th letter of the Greek alphabet, the 19th star in a constellation and a type of unstable subatomic particle.

The baby's age was not disclosed.

The owner of X, formerly known as Twitter, and founder of SpaceX and Tesla already has two children with Grimes, 35, whose real name is Claire Boucher.

Son X Æ A-Xii Musk was born in May 2020, and daughter Exa Dark Sideræl Musk arrived in December 2021 via surrogate.

They call them X and Y for short, which are also the names of electric cars made by Musk's firm Tesla.

The couple - who have an on-off relationship - have so far announced the arrival of their children several months after their births.

The biography on Musk - titled Elon Musk - was written by American writer and journalist Walter Isaacson who shadowed the tech boss for two years.

Musk is a father to 11 children, but lost his first-born child from sudden infant death syndrome (SIDS) when he was just 10 weeks old.

He has been married three times, twice to Westworld star Talulah Riley, and once to Canadian author Justine Wilson.