Kamiyah Mobley, who was taken in July 1998, was found after a tip, the BBC reports.
Authorities in Walterboro, South Carolina, arrested Gloria Williams, 51, in connection with the abduction.
Mobley was living under another name and believed Gloria Williams to be her mother. The biological family have been notified of the news.
Jacksonville Sherriff’s Office said DNA tests had confirmed Mobley’s identity.
She appeared in good health, a “normal 18-year-old woman”, it said.
The office said it had acted on 2,500 tips since the abduction and received one last year to @MissingKids that eventually broke the case.
It released the original composite images released at the time of the kidnapping, along with an image of Ms Williams.
Although the biological family has been notified, the Jacksonville Sherriff’s Office said: “It is up to the victim on how contact will be made. The victim is now an adult.”