The US urged the Turkish authorities to immediately release NASA scientist Serkan Golge, AP reported.
Turkey sentenced Golge in February on charges of belonging to an outlawed group that Turkey blames for attempting a coup that failed in 2016.
The US State Department said it will continue to monitor the Golge case and welcomed the decision of the court to reduce the term of imprisonment.
Earlier, the court reduced the term of imprisonment from 7.5 to 5 years, and orverturned his conviction for membership in a terror organization.
A 36-year-old US citizen, was arrested during a failed coup d'état.