Webster, 34, had been convicted in April by a Hamilton County, Ohio, jury of four counts of unlawful sexual contact with a minor, WXIX-TV, Cincinnati, reported.
Hamilton County Prosecutor Joe Deters said Webster began having sexual contact with the victim, the daughter of a former Bengals coach, when she was 15 and said he was charged only for incidents that took place when she was 15. In Ohio, 16 is the legal age of consent.
During some of the incidents, Deters said, Webster had a gun and threatened to harm the girl and her family if she told anyone.
The victim told her parents about the abuse, WXIX said.
"It is disgusting that Webster would threaten this young victim and her family to keep her from reporting his sexual assaults on her," Deters said.
Webster's attorneys had argued during the trial the victim was 16 years old at the time of the sexual contact.
Webster, who was drafted by the Tampa Bay Buccaneers in 2000, was traded to the Bengals in 2004 and signed as a free agent with the Denver Broncos in 2006. He hasn't played in the NFL since 2008.
Webster was hired last year as the defensive coordinator for the football team at Bellevue High School in Ohio.