The HagueMahamat Said Abdel Kani was found guilty on 4 counts of crimes against humanity.
He is the first Seleka member the court has convicted over Central African Republic violence.
He and others beat and tortured prisoners in a plan targeting perceived supporters of ousted president François Bozizé, judges found.
Judges dismissed several war crimes charges, finding the armed conflict had already ended.
A separate hearing will decide his sentence, and he denied all charges.
Sources: Rappler