The 26-year-old suspect, who has German and Tunisian citizenship, was suspected of having joined Islamic State during a stay in Syria from May to August last year, the federal prosecutor general said in a statement.
The man is accused of receiving combat training in Syria to join Islamic fighters and of retrieving the wounded during a military offensive, the prosecutors said.
There were no indications, however, that the suspect, identified as Ayub B, had concrete plans for an attack, the statement read.
Like other west European countries, Germany is struggling to stop the radicalisation of young Muslims, some of whom want to become jihadist insurgents in Syria or Iraq. Officials also worry that they might return to plot attacks on home soil.
German intelligence authorities estimate that at least 550 people have left Germany for Syria and around 180 have returned. Many are under criminal investigation.
Last week's deadly attacks on French satirical magazine Charlie Hebdo and a Jewish supermarket in Paris that killed 17 people has fuelled fears of assaults on similar targets in other European countries.