The Americans were in control throughout as they recorded their third straight win over the ROC in Olympic competition after previous victories in 2010 and 2018.
Goalkeeper Nicole Hensley was making her second appearance in goal for Team USA, her only other game being the 2018 win, which also ended 5-0.
The Americans are on six points in second place in Group A while the ROC stay at the three points, both teams playing for seeding in the quarterfinals.
"I thought we played really well", Team USA coach Joel Johnson said. "I thought we generated possession. We generated a lot of shot opportunities, but a combination of good goaltending and the style that they played limited our chances.
"We just had to trust the process and hope that we could eventually find some high-quality scoring chances and pucks to the net, and we got that in the third."
Story of the game
Team USA came out aggressive from the start, captain Kendall Coyne-Schofield, skating into the zone and trying to deke ROC goalkeeper Maria Sorokina, but she was able to make the save.
The puck was constantly in the ROC zone and the Americans finally made the pressure pay off when a neat back pass across the crease by Hilary Knight found Harmon, who had the simplest of tap-ins.
The ROC managed just two shots in the opening 20 minutes, but their best opportunity came when Valeria Pavlova blocked a shot at her blue line and raced up the ice, but Cayla Barnes stripped her of the puck before she could get a shot off.
Coyne-Schofield was stopped on a deke in the second period, but Harmon had a hand in the second goal. Her shot from the point was tipped in front by Knight, who beat Sorokina up high.
Hensley was tested a bit, making a pad save off of a shot from the slot by Polina Luchnikova. Later in the second, a two-on-one ended when Viktoria Kulishova had her effort denied by Hensley.
In the third, the ROC started on the power play, but couldn't find a way through Hensley ad although they had good puck movement after that, they failed to convert a couple more times.
A defensive lapse made it 3-0. The ROC failed to clear the zone and after Dani Cameranesi chased down the puck in the zone, she found Zumwinkle with a pass in front and she tipped it in.
Two minutes later, Compher used an ROC defender as a screen and off the rush, she beat Sorokina low to the stick side. Three minutes after that, Sorokina saved Amanda Kessel's shot, but Carpenter knocked in the rebound.
Daria Gredzen replaced Sorokina and stopped all 14 shots she faced, including a breakway by Knight. In the end, it proved to be meaningless as the Americans continued a march towards a second straight Olympic title.
"The difference was the speed", ROC coach Yevgeni Bobariko said. "We did everything we could. I said a lot of good words to the girls in the locker room, and our goalkeeper kept us in the game as long as she could."