Ronaldo, who is widely regarded as one of the greatest players of all time and plays for the Italian club Juventus, has maintained he is innocent of rape.
The allegation made headlines last year after Las Vegas police said they were investigating it.
The case stems from a June 13, 2009, call from a woman to Las Vegas police. She reported that she had been sexually assaulted and police took her to the hospital for an examination, the Clark County District Attorney's Office said in a statement.
But she declined to identify the person who she said assaulted her.
The woman and Ronaldo reached a civil settlement in 2010 and law enforcement did not hear again from her until August 2018, when she asked police to reopen the investigation. She said Ronaldo was the person who sexually assaulted her, 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," the District Attorney's Office said in a statement.
Ronaldo's attorney, Peter Christiansen, and a representative for his sports agents could not immediately be reached for comment.