The baby — Lucas — was found in a suburb of the eastern city of Nancy, a day after a young woman disguised as a hospital worker snatched him, sparking a nationwide alert.
"He was found in the home of a young woman who looks exactly like the person" captured on surveillance cameras, Nancy prosecutor Thomas Pison said.
"She said she gave birth a day ago, but there's no sign of a recent birth," he added.
The unnamed suspect is 17, officials said.
Police official Alain Couic, who is leading the investigation, said the girl initially denied the kidnapping but "afterward, she acknowledged the facts."
Aside from the suspected kidnapper, two men and a women who were in the same apartment were also being questioned by police.
Lucas's parents returned to the regional maternity ward and confirmed thebaby was their child.
Authorities had launched an appeal on radio, television and on noticeboards in train stations as well as motorways to try to track down his abductor.
A woman reportedly walked into the maternity ward and made off with the infant, who was wearing blue pyjamas, as his 24-year-old mother slept. Lucas is her second child.
A source close to the case said the kidnapper had posed as a care worker.
Police brought sniffer dogs to the hospital to try to find traces of the stolen child, as other officers studied video footage from cameras in streets and public transport in Nancy.