In one trial, a German-Turkish citizen named only as Kerim Marc B. received a jail term of six years and nine months for swearing allegiance to Isis chief Abu Bakr al-Baghdadi and joining the group in combat.
The 23-year-old from the western city of Düsseldorf had assumed the nom de guerre Abu Zulfikar and fought in a unit of predominantly Bosnian militants in 2013-14 before he was arrested in Turkey in 2015, the court heard.
Prosecutors in the Düsseldorf trial presented photos of B. with an AK-47 assault rifle and hand grenades and said that even in pre-trial detention, he had sought to convert fellow inmates.
Before the verdict was read, the defendant told the judges he had "completely turned my back on the Islamic State [Isis]".
In another trial, four men were sentenced for supporting a Syrian rebel group the court labelled a "foreign terrorist organisation", the hardline Islamist Ahrar al-Sham.
The four men had supplied the militia, which is battling the regime of President Bashar al-Assad, with thousands of second-hand military jackets and boots as well as ambulances, the court in the southwestern city of Stuttgart heard.
They received terms ranging from three and a half years jail to a suspended prison term of one year and nine months.