Botryllophilus kozloffi is described, based on females living in the branchial sac of the zooid of the compound ascidian Clavelina lepadiformis (Müller, 1776) collected at Roscoff, France. The new copepod belongs to subgroup 2 (leg armature formula pattern II) of female morphotype A (urosome 5-segmented) of the genus, and is primarily characterized by the following morphological features: 1) metasome (segmentation indistinct) large and inflated dorsally, anterior prosome strongly curved from left to right, and urosome long cylindrical; 2) on medial margin of mandibular coxa, 2 conical spines each associated with 1 small subconical spine anteroproximally; 3) right leg 1 exopod approximately trapezoidal, bearing 7 long spines (I–VII) on wide distal margin, its central distal portion bearing 4 spines (II–V) distinctly protruded; and 4) leg 5 short stout, directed posterolaterally.