Phaeosaccion is a genus of algae with monostromatic tubular to saccate thalli, up to 20 centimetres (7.9 in) long and to 2 centimetres (0.79 in) wide. It is the sole genus in the family Phaeosaccionaceae.[1] It is olive brown and resembles young plants of Scytosiphon.[1] The sole species in the genus is Phaeosaccion collinsii, a species of marine algae.[2][3] It was first identified in a publication by W.G. Farlow in the article Notes on New England algae published in Bulletin of the Torrey Botanical Club in 1882.[2] It was named in honor of Frank Shipley Collins.[3]: 56 Phaeosacchion collinsii is red listed in Iceland as a vulnerable species (VU).[4]