The 36-year-old South African former Olympic swimmer and her husband issued a statement expressing their "immense joy" at the news.
The short statement says that the birth is expected before the end of the year. Although it does not elaborate, this implies that the style-conscious princess is at least three months pregnant.
The news is sure to please the palace and end recent tabloid media speculation about why Charlene, after nearly three years of marriage with 56-year-old Albert, had not yet provided an heir to the jet-set European principality. Britain's Kate Middleton became pregnant with Prince George around a year and a half after she married Prince William in April, 2011.
Until the baby is born, the Monegasque heir remains Princess Caroline - Albert's older sister - and then her eldest son, Andrea Casiraghi, who's recently become a father.
The Monaco baby news is likely to set off a frenzy in the fashion press as to how she will dress her bump. Demure Princess Charlene is known for her elegant fashion style, patronising designers such as Giorgio Armani, who designed her couture wedding dress.
Since her marriage, Charlene is styled as Her Serene Highness The Princess of Monaco. The previous holder of the style was her husband's mother, Grace Kelly, who died in 1982.