Brunson was taken from his house -- where he had been under house arrest since July 25 -- under strict security measures and brought to the prison and courthouse complex in Aliağa district for the trial.
On July 25, citing Brunson's health problems, an Izmir court ordered him moved from jail to house arrest.
Brunson's charges include spying for the PKK -- listed as a terrorist group by both the U.S. and Turkey -- and the Gülenist Terror Group (FETÖ) -- the group behind the defeated coup attempt in Turkey of July 2016.
Turkey and the U.S. have faced rocky relations following Washington's imposition of sanctions over Brunson's detention.
Political tensions between the two countries sparked worries in the markets after U.S. President Donald Trump threatened to use economic pressure to secure Brunson's release.