A district attorney in the state of Nevada said it would not prosecute the Portuguese soccer player because it "cannot be proven beyond a reasonable doubt" that a sexual assault occurred.
Kathryn Mayorga, a former model, alleges that Ronaldo raped her in a Las Vegas hotel in 2009.
The former Manchester United and Real Madrid forward reached a financial settlement with her one year after the incident.
He has always denied allegations of sexual assault, insisting their encounter was consensual.
The Clark County District Attorney's Office said it had received a report from Mayorga -- who it identified only as "V" -- on June 13, 2009, reporting that she had been the victim of a sexual assault.
Police could not conduct "any meaningful investigation" at the time because she declined to say who assaulted her or where it occurred, the office said in a statement.
As a result, detectives were "unable to search for and impound vital forensic evidence," and video showing Ronaldo and Mayorga together was lost, it added.
Mayorga contacted police in August of last year to ask that the case be reopened, at which point she named Ronaldo as the alleged assailant.
The case was reopened "in spite of the passage of over nine year," prosecutors said.
"Based upon a review of the information presented at this time, the allegations of sexual assault against Cristiano Ronaldo cannot be proven beyond a reasonable doubt," prosecutors said. "Therefore, no charges will be forthcoming."