A new interstitial hydroid, Sympagohydra tuuli, has been collected from the three-dimensional brine channel system of the coastal fast ice off Barrow, Alaska (71°N 156ºW) in 2003 to 2006. This is the first cnidarian species described from the sea ice interstitial habitat. A morphological and systematic account of the new species is provided here. The hydroid stage is represented by solitary naked polyps, which become almost spherical under contraction. Body length can vary from 200 µm to >1 mm according to the degree of relaxation. The aboral side is extensible into a tubular foot-like projection made by epidermal tissue only, a typical feature of representatives of Protohydridae, order Capitata. However, in contrast to the other two known Protohydridae species, S. tuuli is characterized by three to four solid filiform tentacles, armed with microbasic mastigophore and desmoneme cnidocysts, and located at the base of a short hypostome bearing stenoteles. A small number of mastigophores are also scattered along the body column. Due to its unique combination of features, this taxon is designated here as a new species, and the new genus Sympagohydra is established as a new member of the class Hydroidomedusa, subclass Anthomedusae, order Capitata, family Protohydridae, to accommodate S. tuuli.