Arni () is a former municipality in the Karditsa regional unit, Thessaly, Greece. Since the 2011 local government reform it is part of the municipality Sofades, of which it is a municipal unit. The municipal unit has an area of 89.185 km<sup>2</sup>. Population 2,140 (2021). The seat of the municipality was in Mataragka. It was named after the ancient Aeolian town of "Arne" (), which was located near present Mataragka.