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?
According to her Facebook page, Ms Snell is a native of Daytona, Florida, and had been living in Istanbul.
The American describes herself on her Twitter page as a video journalist.
She also has worked as a senior foreign correspondent and producer for Vocativ since March 2014, according to her LinkedIn page.
Ms Snell also attended University of Florida and Fordham University's School of Law, according to her social media profiles.
What was she doing in Syria?
Ms Snell said in her most recent Facebook post on 5 August that she was held in a cave prison by militants even though she was given permission to film in their territory in Syria.
"I must apologise to my friends and colleagues for all the pain and worry this caused you. I love you all, and I appreciate every effort made to secure my release," her post said.
Ms Snell, who identified herself as Muslim, said she had been staying with the family of one of the militant group's recent martyrs when she was "arrested."
"Because of my unique situation, I was able to convince my captors to give me the use of a phone...which ultimately let me plan my escape, but which also let me document much of my captivity in photos and video," she continued.
Two days later, she was taken into custody by Turkish authorities.
What has Turkey said about it?
According to the Turkish pro-government newspaper Yeni Safak, Ms Snell was detained near the country's Syrian border.
"A US journalist was captured while she was trying to cross the border illegally; she was taken to court and remanded. The trial phase is ongoing. For now, we do not know if she is a spy or not," Hatay Governor Ercan Topaca told national media outlet Anadolu Agency.
A state department official told the BBC it is unclear why she travelled to Syria and whether her profession as a journalist had anything to do with her arrest.
Ms Snell's arrest comes amid strained relations between the US and Turkey after Turkish forces targeted Kurdish fighters in Syria.
The US has criticised Ankara for pursuing Kurdish forces, whom Turkey considers terrorists, along with the so-called Islamic State inside Syria.
Why is the US-Turkey relationship so important?
The US depends on Kurdish forces for supporting in the battle against IS in northern Syria.
The country is also reeling after a failed coup against President Recep Tayyip Erdogan on 15 July.
At least 246 people died during the attempted coup and more than 2,000 others were injured.
President Erdogan has accused the US-based cleric Fetullah Gulen of orchestrating the army-led attempted coup. Mr Gulen has denied any involvement.
Turkish authorities have also cracked down on media outlets in recent weeks, shuttering 131 media organisations in the wake of the failed coup.
Many of the media outlets are linked to Mr Gulen.