Mr Trump, making his first run for public office, praised himself for not attacking Mrs Clinton about the marital infidelity of her husband, former president Bill Clinton, during the debate at Hofstra University.
But he said in a morning interview with Fox News he may take up the attack line going forward.
"I may hit her harder in certain ways," Mr Trump said. He added that when Mrs Clinton criticised him for his treatment of women, he resisted.
"I was going to hit her with her husband's women. And I decided I shouldn't do it because her daughter was in the room," he said. Mrs Clinton brushed off Mr Trump's promise, saying: "He can run his campaign however he chooses."