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 juncifolius is a South African endemic species the Western Cape Province where it can be found from Cederberg to the Kogelberg, Kleinrivier and western Riviersonderend mountain ranges. It is normally found growing in seeps, streamsides, drainage lines and marshy ground. It was previously known as Anomatheca calamifolia, Freesia juncifolia, Ixia zeyheri, Lapeirousia juncifolia, Morphixia juncifolia and Watsonia juncifolia. It grows to 40cm tall with slender leaves. From summer to autumn it bears a lax spike of blue to mauve flowers that are white in the throats.