A judge agreed to dismiss allegations by Lucia Evans, who was one of three female accusers in the criminal case against the 66-year-old.
Weinstein attended a New York court hearing on Thursday while forced to wear a GPS monitor and on $1m (£757,000) bail.
"This is obviously a very positive development," the father-of-five's defence lawyer Ben Brafman told reporters outside court.
In a story published by The New Yorker magazine a year ago, Ms Evans alleged Weinstein forced her to perform oral sex during a meeting at his office in 2004 when she was a 21-year-old student.
Ms Evans' lawyer Carrie Goldberg claimed prosecutors' decision not to oppose the dismissal of the charge had seen them "abandon" her client.
She said: "Let me be clear: the decision to throw away my client's sexual assault charges says nothing about Weinstein's guilt or innocence.
"Nor does it reflect on Lucia's consistent allegation that she was sexually assaulted with force by Harvey Weinstein.
"It only speaks volumes about the Manhattan district attorney's office and its mishandling of my client's case."
Remaining charges in the criminal case include claims Weinstein raped a woman in 2013 and forced a sex act on a different woman in 2006.
Weinstein denies assaulting anyone.
More from Harvey Weinstein
The anger at Harvey Weinstein has been turned into something positive, says London Film Festival director
Sky News tracks down smirking Harvey Weinstein
Harvey Weinstein: Has his fall from grace and #MeToo changed anything?
Gwyneth Paltrow says Brad Pitt 'scared' Harvey Weinstein
Cara Delevingne reveals why she waited to speak up about sexual abuse
World exclusive: Video shows Harvey Weinstein behaving inappropriately with businesswoman
Assistant district attorney Joan Illuzzi-Orbon said the rest of the prosecution's case was strong, telling the judge: "In short, your honour, we are moving full steam ahead."