Dictyostelids are aggregative multicellular (AGM) amoebae, whose life cycle alternates between solitary feeding and multicellular fruiting. The 170+ described species comprise class Dictyostelia, by far the oldest and most diverse AGM lineage with the evolutionary depth of Vertebrata. The best-known species is the model organism, Dictyostelium discoideum. However, many traits studied in Dic. discoideum differ across the class, so a robust and comprehensive phylogeny is required to understand trait evolution. We constructed a dictyostelid-wide phylogeny with 172 deduced protein sequences using published data and 13 new proteomes. Phylogenetic analysis of the data with a diverse Amobozoan outgroup produces a fully resolved and rooted tree that confidently places four poorly studied ”orphan” taxa among its deepest branches. Thus, Synstelium polycarpum and Rostrostelium ellipticum appear as the deepest branches in Heterosteleaceae, supporting parallel evolution of the undifferentiated ”acytostelid-like” morphology. Speleostelium caveatum is confirmed as the deepest branch in Raperosteleaceae, and Coremiostelium polycephalum is sister to both Dictyosteleacea and Raperosteleaceae requiring the erection of a new family, Coremiosteliacaea. We also erect genus Porphyrostelium to accommodate the highly diverse collection of isolates previously grouped together as Dictyostelium purpureum.