In a New York Times article, she says Weinstein pushed her down and "tried to expose himself" at the producer's hotel room in London during the 1990s, before she managed to "wriggle away".
Harvey Weinstein's spokeswoman said the claims about an assault "are untrue".
The 47-year-old star also said she was forced into sex as a teenager by an unnamed actor 20 years older than her.
Thurman had expressed anger at Weinstein last November, saying: "I'm glad it's going slowly - you don't deserve a bullet."