This is the first child for the Princess and her husband Prince Albert, who kept a protective arm around his wife throughout the day.
Wearing a floaty white dress with an on-trend sheer panel at the bottom, Princess Charlene was glowing as she met dignitaries at the event.
Joining the happy couple were Prince Albert's niece and nephew, Pauline Ducruet and Louis Ducruet, the children of his sister Princess Stéphanie.
The Prince's Palace of Monaco released a statement in May confirming the pair's happy news.
The royal couple confirmed the happy news last month. "T.S.H the Prince and the Princess of Monaco are delighted to announce the pregnancy of H.S.H. Princess Charlene," read the announcement.
"The birth is expected at the end of the year."
It was only the second outing for the South African princess after she announced her first pregnancy at the end of May. In June the mum-to-be made an appearance at Fontvieille Park and the Princess Grace Rose Garden, where Prince Albert was tasked with cutting the ceremonial ribbon at the entrance to the grounds while his wife unveiled a plaque commemorating the occasion.
Is Princess Charlene pregnant with twins?
Almost exactly a year ago, Charlene spoke about her desire to start a family. "Now I'm settled and I think kids will come. I don't put any pressure on myself," she told South African newspaper Sunday World. "If it happens, it happens."
In an exclusive interview with HELLO! magazine's sister publication, HOLA! in April, the former Olympic swimmer spoke of her husband as her "biggest support".
"My husband is my biggest support, the person that I confide in and who knows me best," she said