Baby Raphael, a first child for unmarried Charlotte and a second for her Franco-Moroccan partner Gad Elmaleh, was born on Tuesday evening at Monaco's Princess Grace hospital.
"Charlotte Casiraghi and Gad Elmaleh are delighted to announce the birth of their son, Raphael, born in Monaco," said a statement from the palace.
"The grandmothers are doing well," the couple added with a touch of humour.
Charlotte, 27, is the granddaughter of the late Prince Rainier III and Princess Grace, the former Hollywood star Grace Kelly, whose fairytale 1956 wedding helped catapult Monaco from a sleepy Riviera town to a watering hole for the world's rich and famous.
Her uncle, Albert II, is the current Monaco monarch and her mother Caroline is second in line to the throne.
A hugely popular comedian and actor of Moroccan Jewish origin, Elmaleh, 42, has a 12-year-old son, Noe, with French actress Anne Brochet.
The new baby is Princess Caroline's second grandson. Her son Andrea and South American heiress Tatiana Santo Domingo had a baby boy in March.
Charlotte, a keen horse-rider, is Caroline's daughter from her second marriage to Stefano Casiraghi, an Italian industrial heir killed in a speed-boat racing accident in 1990.
She is not a princess like her mother but has inherited her glamorous looks and sense of style, which have earned her a role as a face of Gucci's riding collection.
And despite a relatively discreet private life – by the turbulent standards of the Monaco royals – she is a regular fixture in glossy magazines across Europe.
Long-rumoured, Charlotte's relationship with Elmaleh was made public in March, when he accompanied her to Monaco's Rose Ball, the highlight of the principality's social season.
Since then there has been regular speculation about a possible marriage and Elmaleh recently told People magazine that might happen "one day" while scotching suggestions that nuptials were imminent.
"My mother called me and said, 'I read that you are going to get married on Sept. 14,' and I said, 'What?'," he told the US magazine. "My parents were freaked out. … We aren't married and not engaged."
The comedian added that Charlotte had all the qualities required for her new role.
"She will be the best in the world because she is generous and very loving. She has all the gifts for being a mother. She's precious and very loving and kind."