Platanthera ephemerantha is a species of orchid known by the common names whiteflower rein orchid, slender white piperia, and white-flowered piperia.

It is native to western North America from Alaska to the San Francisco Bay Area, where it grows in coniferous forests and other habitat in coastal and inland mountain ranges within 150 kilometers of the coast.

It grows erect to about half a meter in maximum height from a bulbous caudex.

The fragrant, honey-scented flowers are whiter than those of other Platanthera,[1] but sometimes green-tinged or -veined, or green with white margins.

The status of this species in the wild is difficult to determine because most populations are small and may produce flowers only rarely.