Colpomenia borea sp. nov. is described from Hokkaido, Japan and Magadan, Far East Russia based on morphological observations and molecular analyses using mitochondrial cox3 and chloroplast rbcL genes. This new species is distinguished from other Colpomenia by its small globular to ovoidal thallus up to 5 cm in diameter, and thin thallus membrane composed of a cortex of one to two-cell layers and a colorless medulla of up to three layers. This species is epiphytic on the brown alga Stephanocystis in areas protected against waves. The life history in culture of C. borea was investigated and the observed life history pattern was similar to those reported in C. peregrina and C. sinuosa. Our phylogenetic analyses supported that C. borea is a distinct species, yet it is more related to C. peregrina than C. sinuosa.