Söderfjärden is a polder in Ostrobothnia, western Finland, 10 kilometres south of the town of Vaasa.

The plain is in an impact crater which was made at least some 640 million years ago (Proterozoic, near the end of the Cryogenian).

The crater's diameter is 6.6 kilometres (4.1 mi) and its maximum depth is 300 metres (980 ft).

After rising from the sea due to post-glacial rebound, Söderfjärden was a wetland but was later drained with help of a pump station.

This makes Söderfjärden unique among the other impact structures in Finland which are at least partially below sea level.