Every year numerous ecological, biochemical, and physiological studies are performed using members of the order Laminariales. Despite the fact that kelp are some of the most intensely studied macroalgae in the world, there is significant debate over the classification within and among the three “derived” families, the Alariaceae, Laminariaceae, and Lessoniaceae (ALL). Molecular phylogenies published for the ALL families have generated hypotheses strongly at odds with the current morphological taxonomy; however, conflicting phylogenetic hypotheses and consistently low levels of support realized in all of these studies have resulted in conservative approaches to taxonomic revisions. In order to resolve relationships within this group we have sequenced over 6000 bp from regions in the nuclear, chloroplast, and mitochondrial genomes and included 42 taxa in Bayesian, neighbor-joining, and parsimony analyses. The result is the first comprehensive and well-supported molecular phylogeny for the ALL complex of the Laminariales. We maintain the three recognized families (Alariaceae, Laminariaceae, and Lessoniaceae), but with vastly different compositions, as well as propose the Costariaceae fam. nov. for Agarum, Costaria, Dictyoneurum, and Thalassiophyllum, the only genera in the Laminariales with flattened, occasionally terete, stipes and either a perforate or reticulate blade. In addition, our data strongly support a split of the genus Laminaria. We resurrect the genus Saccharina Stackhouse for the Laminaria clade that does not contain L. digitata (Hudson) J.V. Lamouroux, the type of the genus.