Pilularia globulifera, or pillwort, is an unusual species of fern native to western Europe, where it grows at edges of lakes, ponds, ditches and marshes, on wet clay or clay-sand soil, sometimes in water up to 30 cm (12 in) deep. It has a pea-shaped 4-chambered sporocarp, each chamber formed from a modified leaf and containing several sori bearing both macrosporangia and microsporangia. The species is thus heterosporous.

Some of the plants growing in association with this species in the UK include water celery (Apium inundatum), marsh pennywort (Hydrocotyle vulgaris) and lesser spearwort (Ranunculus flammula).

This is an internationally threatened species included in the European Red Data Book. It is listed on Schedule 8 of the Wildlife (Northern Ireland) Order 1985, but it has not been seen since 1970 and may now be extinct in the province. It is protected under the Wildlife and Countryside Act 1981 in the rest of the UK, where it is now classified as Vulnerable.

Pillwort can be grown in a "bog garden" or as a marginal aquatic in a garden pond.