"I think it'd be very hard to deal if they do violence, I mean, if it's another Tiananmen Square," Trump told reporters in New Jersey on Sunday. "I think it's a very hard thing to do if there's violence."
Hong Kong has been roiled by protests for more than two months and on Sunday hundreds of thousands of democracy activists had marched peacefully in the city despite rising unrest and stark warnings from Beijing.
There have been increasingly stark warnings from Beijing over the protests, and state media has run images of military personnel and armoured personnel carriers across the border in Shenzhen.
China's 1989 crackdown in Beijing's Tiananmen Square saw tanks sent in to end student-led protests, resulting in hundreds if not thousands of deaths.
Trump's comments came as Washington and Beijing look to revive pivotal talks aimed at ending their trade war, which has roiled world markets.