Charles, 65, made his pronouncement when chatting to a well-wisher at an event for his charity, the Prince's Trust, on Tuesday.
Corey Johnson, 36, had passed on his congratulations for the prince's again becoming a grandfather, and Charles replied, "I am already!"
Then, according to Johnson, he added: "But it's wonderful to have another one. I'm happy I'm going to be a grandfather again. I'm looking forward to it – but I hope it will be a girl this time."
At the same event – to highlight entrepreneurs who have been helped by the Prince's Trust in south London – Charles told Toying Owoseje, 26, of his joy over the pregnancy news.
"It's wonderful," he said. "We are very happy. It's splendid, very exciting."
When she was pregnant with baby Prince George (now 13 months) in March 2013, Kate, 32, said that husband William wanted a girl, and that she was hoping for a boy.
William's mother, Diana, Princess of Wales, told biographer Andrew Morton in his 1992 book, Diana, Her True Story, that Charles always favored a girl when they were expecting their second child. That infant turned out to be a boy, Prince Harry.
"Charles always wanted a girl. He wanted two children and he wanted a girl," Diana said. "I knew Harry was a boy and I didn't tell him."