Gabriella was born first at 5.04pm, and Jacques at 5.06pm.
Their arrival was announced by the firing of 42 shots from the Royal cannon with details of the birth revealed via the very modern medium of Facebook, which added: "The Princess and the children are doing well."
Gabriella and Jacques are the first children for the 36-year-old former South African Olympic swimmer, who married Prince Albert II, 56, three years ago.
And despite being born second, Jacques will be the future Prince of Monaco, because of the Mediterranean principality's male inheritance laws.
The first reports of the royal birth emerged in local paper Monaco Matin, followed by further articles in Dutch broadsheet, De Telegraaf.
Albert, the son of the late American actress Princess Grace, had worried some of his subjects with his long bachelorhood and his lack of an heir.
He married Charlene Wittstock, a Zimbabwe-born, South Africa-raised former Olympic swimmer in 2011. Her pregnancy was revealed in October.
Ahead of the birth, Monaco resident Isabelle Roux, said: "This is going to create an immense joy. Immense!
"They are awaited like the messiah ... Everyone is talking only about that."
Adelaide de Clermont-Tonnerre, editor-in-chief of the celebrity weekly Point de Vue, added: "Two babies for the price of one. I think it's very good for the image."
The babies will be the first twins in the royal household, which was established in the 13th century, but they will not share the throne.
Had they both been of the same sex, the first one out would have taken the throne, but because the Monégasque twins are a boy and a girl, the boy will automatically inherit.
He will be given the title of Marquis, although his sister's title is yet to be decided as the one given to a second son - Earl – can't always be given to girls.