Epilobium alsinoides is a variable, small, willow herb, growing to 6 in (15 cm) high. The tiny leaves are tiny and more or less toothed. Small, single, white flowers develop near the branch ends.
Epilobium alsinoides is found in tussock grassland, herbfields, riverbeds and open well-drained sites, lowland to low-alpine, North Island, South Island and Stewart Island, New Zealand. Three subspecies are recognised.