The 'Hangover' star, 38, fuelled speculation that he is seeing the 20-year-old British beauty when the pair was spotted enjoying a day out sightseeing in Boston, Massachusetts, reported People magazine.
Cooper was first seen in public with Waterhouse last month when they left the Elle Style Awards in London together in the same car.
"It's new but they're hanging out. He's really into her," a source said.
Cooper, who is in Boston to shoot a new movie with his 'Silver Linings Playbook' director David O Russell, has been single since his on/off romance with 'Avatar' star Zoe Saldana ended shortly before Christmas.