The female of Botryllophilus sarsi is described as a new species, replacing B. brevipes Sars, 1921, which is a junior homonym of B. brevipes Brément, 1909. The description is based on specimens on loan from the Zoological Museum, University of Oslo. These copepods, living in a polycitorid ascidian, were collected on the west coast of Norway in 1937 and had been identified as B. brevipes Sars. Botryllophilus sarsi belongs to subgroup 1 of morphotype A of the genus and is distinguished from its five congeners by a combination of minor morphological features of body form, antennule, legs, and genitalia. Comparison of B. sarsi with similar botryllophilins and consideration of their ascidian hosts may lead us to question whether the host for B. brevipes Sars is a botryllid.