Harrison defeated Britain's Gemma Gibbons in a final she dominated from the start, getting a stronger grip on Gibbons and managing to throw her twice.
It was the second American medal in judo this week: teammate Marti Malloy won a bronze in the women's 57-kilogram division.
Harrison, 22, is the world No. 4 in her division and a previous world champion. She was in top form on Thursday, dispatching many of her opponents with a match-ending ippon, one in less than a minute.
The bronze medals were won by Audrey Tcheumeo of France and Mayra Aguiar of Brazil.