-- The Force is strong with them.
Fisher, who writes about the affair in her new memoir "The Princess Diarist," says it took place over three months while filming "A New Hope" in 1976.
“It was so intense,” she tells People magazine. "It was Han and Leia during the week, and Carrie and Harrison during the weekend."
Fisher was 19 at the time, while Ford was 33.
A rep for Harrison Ford did not immediately return ABC News' request for comment about Fisher's claims. She tells People he knew about the upcoming book.
The two recently reunited on-screen again as Leia and Han in last year's "The Force Awakens," which also had an appearance from Mark Hamill's Luke Skywalker. In the film series, Han and Leia fall in love and eventually have a son.
About their romance four decades ago, Fisher adds, "I was so inexperienced, but I trusted something about him. He was kind.”