Her husband, Prince William, however, would prefer a girl.
The duchess, who is five months pregnant, made the revelation to a soldier at a St. Patrick's Day ceremony honoring the Irish Guards.
"I asked her 'do you know if it's a girl or boy?' and she said 'not yet,'" said Guardsman Lee Wheeler. "She said 'I'd like to have a boy and William would like a girl.'"
Wheeler said the duchess said she and her husband have yet to settle on names for the baby, who will be third in line to the British throne.
The mom-to-be wore a deep green dress coat and a shamrock lapel pin as she presented sprigs of shamrock to soldiers in the annual ceremony at a barracks in Aldershot, southern England. She also pinned a spring on the collar of the regiment's mascot, an Irish wolfhound named Domhnall.
Kate suffered a minor mishap when one of her high heels became briefly stuck in a drainage grille. The duchess held onto her husband's hand as she wrenched it loose.
The royal St. Patrick's Day tradition was started by Queen Alexandra, the wife of King Edward VII, in 1901.
The Associated Press contributed to this report.