Lindsey Snell was arrested earlier this month, US state department spokesman John Kirby said.
The Florida native recently posted on Facebook that she was kidnapped in July by Jabhat al Nusra, formerly al-Qaeda's Syrian affiliate, and had escaped.
The US said it was in touch with Turkish officials regarding the case.
Ms Snell is being held at a prison in the southern Hatay Province and consular officials had visited her on 26 August, Mr Kirby added.
Who is Lindsey Snell?