Borealea nobilis is a species of sea slug, an aeolid nudibranch, a marine heterobranch mollusc in the family Flabellinidae.
This species is translucent white with opaque white markings. The digestive gland in the cerata is either orange or red in colour. The rhinophores are pale yellow-brown in colour and covered with small papillae.
This species grows to 40–50 mm (4–5 cm) in length. The maximum recorded body length is 63 mm.
Borealea nobilis was described from a single specimen dredged in 145 m depth off Cape Cod in 1879. It is widely distributed in the northern Atlantic Ocean from Norway south to Ireland and on the North American coast south to New England.
Minimum recorded depth is 20 m. Maximum recorded depth is 190 m.
