The prosecutor's office in the southwestern city of Karlsruhe said the 60-year-old German national identified only as Manfred K. was believed to have stolen "state secrets" for unidentified "third parties."
"He is believed to have illegally acquired, as a NATO civilian employee, classified data belonging to his employer at the US armed forces' air base at Ramstein and to have copied it onto his personal computer," it said in a statement.
"It is suspected that this was done with the intention of passing the obtained data on to unauthorised third parties."
A judge ordered the suspect remanded in pre-trial detention Monday and the state criminal investigations office in Rhineland-Palatinate will supervise the ongoing probe, it added.
The prosecutor's office declined to provide further details on who may have sought the data, when it was taken or how sensitive the files were.
NATO spokeswoman Carmen Romero declined to speak about the case.
"NATO does not comment on intelligence matters," she said.