The Ohio 1st District Court of Appeals ruled Wednesday prosecutors didn't prove one of four counts of unlawful sexual conduct with a minor on which Webster was convicted last year. It upheld his other convictions.
Prosecutors say Webster had sex with the girl several times in 2009 when she was 15. Webster says the sex occurred when she was 16, Ohio's legal age of consent.
The Hamilton County prosecutor's office hasn't returned a message seeking comment.
Webster played for the Tampa Bay Buccaneers, the Bengals and the Denver Broncos between 2000 and 2008.