Cnidarian relationships have been a source of debate for decades with almost every possible combination of sister group relationship between major taxa proposed to date. Recent, data-rich, phylogenomic studies using nuclear data have substantially advanced our knowledge of the cnidarian tree of life. However, these studies have either lacked key taxa, or failed to provide adequate support for the position of some key groups. Using both new publicly released and newly-generated DNA and RNA-seq data for representatives of all cnidarian classes, we propose the first fully resolved genomic-based cnidarian phylogeny. Our Cnidaria dataset is the most extensive to date, including sequence-rich datasets for three Staurozoa species and one Ceriantharia. Our trees support the monophyly of Anthozoa, where the cerianthid Cerianthus borealis is a full member of Hexacorallia.Our results confirm earlier findings of the sister relationships between Medusozoa and Endocnidozoa (Myxozoa and Polypodium hydriforme). The relationships within Acraspeda (Cubozoa, Scyphozoa, Staurozoa) revives evolutionary hypotheses put forward more than half a century ago by Hyman (1940) and Thiel (1966), including the Rhopaliophora that regroups cubo- and scyphozoan taxa. A fully resolved phylogeny of major cnidarian clades opens up the ability to study key innovations in cnidarian body plans.