Bacidia caesiovirens S. Ekman & Holien sp. nov. is described from Norway, and the British Isles. It occurs on trunks of deciduous trees in coastal forests. This species is usually sterile and is characterized by its conspicuously blue-green, granular thallus, which contains a bluish, N+ purple pigment. It is closely related to B. biatorina (Körb.) Vain., but differs from this species in a number of features apart from thallus pigmentation: the thallus granules are smaller, the amount of hyphal projections from the surface of the thallus granules is larger, and the internal pigmentation of the apothecia is different. Bacidia auerswaldii (Hepp ex Stizenb.) Mig. has a similarly granular thallus but differs from B. biatorina and B. caesiovirens in having wide, fusiform spores and smaller thallus granules. Bacidia absistens (Nyl.) Arnold has an internal apothecium pigmentation similar to B. caesiovirens, but differs in having a smooth thallus, and an abundance of minute crystals in the excipulum.