Sierre (syĕr) [key], Ger. Siders, town (1990 pop. 14,143), Valais canton, S Switzerland, on the Rhône River. A market town, it is the center of a rich horticultural region. There is a large aluminum plant nearby. Sierre has a 13th-century tower and a 16th-century castle. The poet Rilke lived there.