Dendrogramma was the iconic deep-sea animal of 2014, voted among the top-ten new species described that year. The two species described are mushroom shaped animals, diploblastic, with an apparent gastrovascular system that extends from the base of the stalk to bifurcating canals that radiate through the flat disc. The authors could not assign the new genus to any known animal group with certainty, leading to numerous media reports that it belonged to an entirely new phylum. Here we use phylogenomic data from newly collected specimens to show that Dendrogramma is a cnidarian, specifically a benthic siphonophore in the family Rhodaliidae. Although an entire Dendrogramma colony has not been found, we hypothesise that the mushroom-like bodies are bracts, possibly used to aid buoyancy or as defensive appendages to protect feeding gastrozooids or gonads.