Breynia assimilis is an extremely rare species of flowering plant in the family Phyllanthaceae.

It is a shrub or tree endemic to the wet evergreen lowland forests of southwestern Sri Lanka.

It is only known from the Sinharaja Biosphere Reserve there, evidence of its existence was last catalogued before 1991 (in a survey held between 1991 and 1996 by the National Conservation Review of Sri Lanka), and it has not been found since then.

[1] The species was first collected for scientific examination and classification from Allagalla,[4] in the Central Province of Sri Lanka, at an elevation of 3,000 feet (910 m), by botanist George Henry Kendrick Thwaites, who named it Sauropus assimilis.

[2] This high elevation, when compared with where it has been found more recently, would seem to indicate that the historic range of B. assimilis is much broader than it stands today.