"We have a duty to only file those charges if we have a reasonable likelihood of a conviction," State Attorney Willie Meggs said at a standing room-only news conference. "We did not feel that we could meet that burden. We do not have sufficient evidence to make the charge."
Winston, 19, has led his team to the top of the national polls as a freshman while facing scrutiny over recently surfaced accusations that he assaulted a woman in December 2012, before his college playing career began.
Winston was accused by a Florida State University student who said she was raped after a night of drinking with friends at a bar near campus.
Meggs said the woman had "memory lapses on major issues." He said Winston declined to be interviewed, although he gave a DNA sample.
Winston's attorney, Timothy Jansen, later told reporters that his client had been vindicated and reiterated his earlier claim that Winston had only consensual sex with his accuser.
"It was a consensual encounter between Jameis and a young woman. Two other witnesses were present there," said Jansen. "Clearly, the young woman consented to whatever activity took place," Jansen said.
He denied that the woman had been drunk or intoxicated when the encounter occurred and said "her story just does not add up."
Meggs, who is the state prosecutor for six counties in northern Florida, said repeatedly that Winston's star status and the fanatic importance of FSU football in Florida's Big Bend area did not figure into his decision. "I have not been pressured, nor have I consulted the football schedule. We worked as fast as we could," he said.
The unidentified complainant, who has withdrawn from the university, said in a search warrant affidavit filed by Tallahassee police that she had a few drinks with friends at Potbelly's, a popular bar near campus, on December 7. Her friends left and she said she remembered getting into what she thought was a cab with a man who took her to an apartment and assaulted her.
Winston's DNA was found in the woman's underwear. She did not identify Winston as her assailant until January 10. City police initially marked the case "inactive" but resumed the investigation last month after news reports identified the football star as a suspect.
Meggs said laboratory reports identified a boyfriend as the source of another DNA sample also found in the woman's shorts, but she declined to identify him. He said police subsequently determined the man's identity.
The woman's family issued a statement thanking Meggs for his investigation, despite "an inordinate delay by the Tallahassee Police Department."
The woman's family has been critical of police handling of the investigation. Meggs, who received the police report only after news leaked to the gossip web site TMZ, said "obviously it would have been better if it was handled a little different, a little earlier."
Attorney Patricia Carroll, representing the family, cited weeks of harsh comments made about the woman on social media, some of which purported to identify her.
"The victim in this case had the courage to immediately report her rape to the police and she relied upon them to seek justice," said the statement. "The victim has grave concerns that her experience, as it unfolded in the public eye and through social media, will discourage other victims of rape from coming forward and reporting," it added.
The timing of the charging decision was crucial.
The Florida State Seminoles will play for the Atlantic Coast Conference title on Saturday in Charlotte, North Carolina. If they win, it is likely the team will play for the national collegiate title in the Rose Bowl next month.
A felony charge against Winston would have required his immediate suspension under the school's athletic department policy.
Votes for the Heisman Trophy, the annual award given to the country's best college football player, are due on Monday.