The man identified only as Shaas Al-M. allegedly joined the jihadist group in 2013 and took part in various military operations in Syria, they said in a statement.
Prosecutors declined to tell AFP when and how the suspect entered Germany last year, which in 2015 took in nearly 900,000 asylum seekers, around 40 percent of them from war-ravaged Syria.
Once he arrived, "he inspected potential attack targets during stays in Berlin and arranged to send at least one person to Syria as a fighter".
"In addition, he served as a contact man for potential attackers in Germany and indicated he was prepared to carry out an attack in Germany," they added.
The suspect was arrested in March after prosecutors in Potsdam near Berlin laid criminal conspiracy charges.
He stands accused of membership of a foreign terrorist organisation as well as military weapons law violations.
Germany has so far been spared large-scale jihadist attacks.
But Europe's biggest economic power has been shaken by two assaults claimed by IS and carried out by asylum seekers -- an axe rampage on a train in Würzburg that injured five, and a suicide bombing in Ansbach in which 15 people were hurt.
Police said this month they had foiled an alleged plot by a Syrian refugee to bomb one of Berlin's airports.