Kate will be the guest of honour at the 100 Women In Hedge Funds Gala benefiting Action on Addiction on 24 October, it was confirmed by Clarence House today.
The glitzy event will be held at The State Apartments at Kensington Palace.
The 100 Women In Hedge Funds is a global association of professional women which aims to make a difference in industry and the community through education, philanthropy and leverage initiates.
Action on Addiction, meanwhile, is a charity that works in the fields of research, prevention, treatment, education and support for families and people affected by addiction.
The Duchess is a patron of both organisations.
Kate's appearance at the event will come the day after the christening of her and Prince William's son Prince George Alexander.
The Archbishop of Canterbury, Justin Welby, will christen the third-in-line to the throne at the Chapel Royal, St. James's Palace on 23 October.
There's no word yet on who the godparents will be.
Kate gave birth to George at the St. Mary's Hospital in Paddington, London on 22 July. His full title is His Royal Highness Prince George Alexander Louis.
The 31-year-old's next public appearance will be 18 October at a SportsAid Athlete Workshop at the Queen Elizabeth Olympic Park in London.
First pictures of the royal baby: