Thereianthus is a small genus of cormous plants from the Southwestern Cape of South Africa in the Iridaceae family. They are unusual for a winter rainfall climate as most species bloom in the dry warm summer months. Flowers are blue, purple, or white.

Thereianthus ixioides is a rare South African endemic species fromWestern Cape Province where it is usually found in Fynbos growing on cool or seasonally damp south-facing slopes at mid altitudes on the mountains between Wellington and Franschhoek. It was previously known as Watsonia filifolia and Watsonia punctata var. filifolia. It grows to 40cm tall with linear leaves. In late spring it bears dense spikes of white flowers with violet markings in the centres.