Jagger, 70, gets the title courtesy of 21-year-old granddaughter Assisi Jackson, who has reportedly given birth to a girl. The child's name has not been made public.
The father is Alex Key, 25, a chef. Jackson has a small homeopathy business, the Mail said.
"I imagine it's nice to be a great-grandad — although I'm not sure he likes the idea of getting old, or being called one," Assisi Jackson told Hello! "I call him Mick — I wouldn't start calling him grandpa."
And it seems Jade Jagger, whose mother is Bianca Jagger, Mick's first wife, hasn't even considered it to be news.
"I think making Mick a grandfather obviously had a big impact," she told the Sunday Times of London back in November, "whereas, now, making him a great-grandfather is no longer particularly interesting."
Not that the family situation is drama-free.
That joint baby shower in mid-March, right around the 50th anniversary of the Stones' first album, was hosted by Mick's former common-law wife, Jerry Hall, who's reportedly been like a stepmother to Jade.
It was Mick's relationship with Jerry that spurred Bianca to file for divorce after seven years of marriage to the rock star, alleging adultery. Mick and Jerry were in a relationship for 23 years, which included an unofficial 1990 wedding in Bali. They split in 1999.